Change the way twig filetype detection works.

Use filetype.vim instead of ftdetect/twig.vim because vim 7.4 comes with
a default autocmd, and we want ours to come first. Set the filetype
using the base file name, and then carefully add ".twig".
Replace "Jinja" with "Twig" in syntax file comments.
Do not assume that the base filetype is HTML
Do something reasonable for foo.twig.twig.