"stack new" allows invalid package names

[mgsloan]

Repro:

1. stack new "1234-abc"

2. Fails to initialize a stack.yaml due to Unable to parse cabal file /home/mgsloan/fpco/stack/1234-abc/1234-abc.cabal: NoParse "name" 1

Looks like the packageNameParser doesn't restrict package names enough. From the cabal users guide:

A package is identified by a globally-unique package name, which consists of one or more alphanumeric words separated by hyphens. To avoid ambiguity, each of these words should contain at least one letter.