Add struct for TE header field

[sector-f]

The TE header field is similar to Accept-Encoding, but for the Transfer-Encoding instead of the Content-Encoding

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/TE
Section 14.39 https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

[almielczarek]

I'm interested in helping with this!

Just to make sure I am on the right track, I think what I should do is implement the TE struct using the header! macro, passing the QualityItem<Encoding> struct as this also has a notion of quality values, adding the Trailer variant to the Encoding enum and adding it to the related impls, and then writing the documentation based off of RFC7230. Does this make sense or am I missing something?