The `gf` command opens files under the cursor. This change configures
the kotlin filetype to enable this command on import statements by
replacing dots and appending the file extension.
The functionality is copied from ftplugin/java.vim as distributed in vim
source code.
This allows us to have C-style comments in kotlin, a la:
```
/**
* Start of comment
*
* More comment
*/
```
and have vim autoindent them (according to your formatoptions setting,
that is). I copied the setting from the builtin ftplugin for
JavaScript, since they have equivalent comment syntax.
