Make endpoint special-case localhost and IP addresses

I believe it's quite common (e.g. for Realtime devs) to want to point to
a local Realtime. Or, for example in the SDK team, to want to point to a
local reverse proxy for interception.

Turns out that it's already special-cased in ably-go [1], so let's add
to spec.

[1] https://github.com/ably/ably-go/blob/17f6575773b28a146ac04bfaf3b8de44ce1ff162/ably/options.go#L506-L510

Author: lawrence-forooghian

textile/features.textile

@@ -75,7 +75,8 @@ h2(#endpoint-configuration). Client library endpoint configuration
 ** @(REC1a)@ The @primary domain@ is @main.realtime.ably.net@ unless overridden by specifying an @endpoint@ option or any of the deprecated options @environment@, @restHost@, @realtimeHost@.
 ** @(REC1b)@ If the @endpoint@ option is specified then:
 *** @(REC1b1)@ If any one of the deprecated options @environment@, @restHost@, @realtimeHost@ or @fallbackHostsUseDefault@ are also specified then the options as a set are invalid.
-*** @(REC1b2)@ If the @endpoint@ option is a domain name, determined by it containing at least one period (@.@) then the @primary domain@ is the value of the @endpoint@ option.
+** @(REC1b2)@ This clause has been replaced by "@REC1b5@":#REC1b5. It was valid up to and including specification version @TBD@.
+*** @(REC1b5)@ If the @endpoint@ option is a hostname, determined by it containing at least one period (@.@), or containing at least one instance of the string @::@, or being equal to the string @localhost@, then the @primary domain@ is the value of the @endpoint@ option.
 *** @(REC1b3)@ Otherwise, if the @endpoint@ option is a non-production routing policy name, determined by it having the form @nonprod:[id]@, then the @primary domain@ is @[id].realtime.ably-nonprod.net@.
 *** @(REC1b4)@ Otherwise, the @endpoint@ option is a production routing policy name of the form @[id]@, and the @primary domain@ is @[id].realtime.ably.net@.
 ** @(REC1c)@ If the deprecated @environment@ option is specified then it defines a production routing policy name @[id]@:
@@ -92,7 +93,7 @@ h2(#endpoint-configuration). Client library endpoint configuration
 ** @(REC2b)@ Otherwise, if the deprecated @fallbackHostsUseDefault@ option is specified then the set of @fallback domains@ is the default set defined in @(REC2c1)@.
 ** @(REC2c)@ Otherwise, the set of @fallback domains@ is defined implicitly by the options used to define the @primary domain@ as specified in @(REC1)@:
 *** @(REC2c1)@ If the @primary domain@ is determined to be the default via @(REC1a)@ then the set of @fallback domains@ is the default @main.a.fallback.ably-realtime.com@, @main.b.fallback.ably-realtime.com@, @main.c.fallback.ably-realtime.com@, @main.d.fallback.ably-realtime.com@, and @main.e.fallback.ably-realtime.com@.
-*** @(REC2c2)@ Otherwise, if the @primary domain@ is determined to be an explicit domain name via @(REC1b2)@ then the set of @fallback domains@ is empty.
+*** @(REC2c2)@ Otherwise, if the @primary domain@ is determined to be an explicit hostname via @(REC1b5)@ then the set of @fallback domains@ is empty.
 *** @(REC2c3)@ Otherwise, if the @primary domain@ is determined by a non-production routing policy name via @(REC1b3)@ then the set of @fallback domains@ is @[id].a.fallback.ably-realtime-nonprod.com@, @[id].b.fallback.ably-realtime-nonprod.com@, @[id].c.fallback.ably-realtime-nonprod.com@, @[id].d.fallback.ably-realtime-nonprod.com@, @[id].e.fallback.ably-realtime-nonprod.com@.
 *** @(REC2c4)@ Otherwise, if the @primary domain@ is determined by a production routing policy name via @(REC1b4)@ then the set of @fallback domains@ is @[id].a.fallback.ably-realtime.com@, @[id].b.fallback.ably-realtime.com@, @[id].c.fallback.ably-realtime.com@, @[id].d.fallback.ably-realtime.com@, @[id].e.fallback.ably-realtime.com@.
 *** @(REC2c5)@ Otherwise, if the @primary domain@ is determined by a routing policy name via @(REC1c2)@ then the set of @fallback domains@ is @[id].a.fallback.ably-realtime.com@, @[id].b.fallback.ably-realtime.com@, @[id].c.fallback.ably-realtime.com@, @[id].d.fallback.ably-realtime.com@, @[id].e.fallback.ably-realtime.com@.