React fragment shorthand syntax support

[LumaLivy]

This solves #258 by adding in support for fragment shorthand syntax.

<>
    <p>Neat.</p>
</>

I've covered adding in the valid grammar as a separate regex check for tag opening and closing because things got weird when I tried to amend the existing tag open and close regex. Additionally I changed the regex in atom-react.coffee to allow auto-completion of fragments, which seems to have no adverse effects from my testing.

Unrelated to this package, bracket-matcher seems to be unable to properly highlight fragment tags due to them missing a tag name. This was the only side effect I noticed.