Pup 0.3.1

Taking inspiration from the structure provided by [Library/Formula/terraform.rb](https://github.com/Homebrew/homebrew/blob/master/Library/Formula/terraform.rb)
this formula builds the `pup` html query utility.
This shouldn't be necessary to specify explicitly, since you're not running `go get`; having a mercurial resource will automatically add a mercurial dependency.