[textual] Add an emacs mode for SIL
[textual] Add an emacs mode for SIL

Summary:
Add an emacs major mode for textual SIL files. The mode currently is
pretty basic and has only the syntax highlighting. Still this is better
than nothing.

Reviewed By: aorenste, jvillard

Differential Revision: D39884080

fbshipit-source-id: 75ebd97d9043438de00033f24b303200aecebbb2

facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer