Custom `React.Fragment` pragma for JSX

[ClassicOldSong]

Now we can customize JSX pragma for React.createElement. But here shows that buble now hardcodes fragment expression as React.Fragment which is not really a good idea. Can we make fragment pragma customizable? Such as adding an option jsxFrag to the option list?

Here we can see babel has already supported customizing fragment pragma. Hope buble can add this soon.

[ClassicOldSong]

I'll see if I can submit a pull request on this

[mourner]

Fixed in #199