* Require language part of locale to be two lowercase letters.
* Update Makefile.doc comment -- add "include" comment.
* Get rid of muttator/locale/en-US/Makefile symlink in favor of include.
At the moment, only "syntax=asciidoc" was specified. The
asciidoc_filetype.vim that is distributed with Asciidoc gives the
installer two options -- to either associate every *.txt file with
asciidoc or to try to decipher whether or not the file is an asciidoc
using the first 50 lines.
So it's not given that a developer will open the *.txt files with
ft=asciidoc, and so it's helpful to have it explicitly in the modeline.
