[WIP] Auto-lint staged files

[shawnbot]

This is a test of the workflow/setup suggested in #253, which uses husky and lint-staged to automatically add and run a git precommit hook that lints staged files before a commit.

So far, I've only tested it by changing a comment and committing it. Beforehand I ran npm run precommit just to see what happened, and that may be when husky added the git hook. After that, I did the following and it Just Worked:

shawnbot@github:~/primer/primer-css% git commit -m 'fix classname in comment' \
    modules/primer-forms/lib/form-select.scss

husky > npm run -s precommit (node v7.10.1)

 ✔ Running tasks for *.scss
[shawnbot/auto-lint bb7d531] fix classname in comment
 1 file changed, 1 insertion(+), 1 deletion(-)

[shawnbot]

Closing this because I think it makes people nervous. We can already auto-lint our stuff individually, and raise errors on CI; no need to be heavy-handed about it.