Enable using '%' to jump to matching tags

This adds a file which enables matchit to be used with JSX tags, using a regex taken from `/usr/local/share/vim/vim74/ftplugin/html.vim` (on my machine).

This means when pressing `%` on the opening or closing part of a JSX tag the cursor will jump to the matching tag. It requires that the user has matchit (which comes with Vim) enabled by putting

```
runtime macros/matchit.vim
```

in their `.vimrc`.