csslint: Add .csslintignore file

Note that the minimatch glob csslint uses doesn't support
"dir" or "dir/", it needs "dir/**" to recursively exclude a
directory:
- "file"
- "unsupported/"
- "dir-contents/*"
- "dir-contents-and-recurse/**"

We're excluding dist/ because there's certain warnings that
only trigger in csslint when there are lots of rules (such
as the warning about "too many usage of floats") which are
being triggered otherewise.

Change-Id: I7d34f07f1d8352261ad97291e215b68da363faba