WAST support on GitHub

Hello!

I meant to give a heads up nearly a month ago, but better late than
never... Just to inform you all that WebAssembly will soon be formally
classified on GitHub <https://github.com/github/linguist/pull/3650>.

This has two (admittedly superficial) benefits:

   1. It will now be possible to search for "WebAssembly" repos on GitHub,
   which should also facilitate gauging the language's in-the-wild use (at
   least as far as .wast and .wat files are concerned).
   2. .wast and .wat files will now receive syntax highlighting using
a language-grammar
   hand-written <https://github.com/Alhadis/language-webassembly> by yours
   truly.

If there are any corrections or updates that should be made to the
grammar's highlighting, please file an issue and I'll fix it as soon as
possible. =)

(The changes will be live on GitHub with the next release of Linguist
<https://github.com/github/linguist>. We don't have a concrete release
schedule, but it generally happens every few weeks or so...)

Regards,
- John / @Alhadis <http://github.com/Alhadis>