Add regex syntax highlighting

[rrudakov]

Close #11

Sample pictures:

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• The commits are consistent with our contribution guidelines.

• You've run M-x checkdoc and fixed any warnings in the code you've written.
• You've updated the changelog (if adding/changing user-visible functionality).
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Thanks!

[bbatsov]

@rrudakov You'll need to rebase this on top of master first.

[bbatsov]

The proposed changes look great!