Feature request: VimRuntime command

I've added a few simple, short commands to my vimrc to enable me to quickly navigate my own vim runtime files, and one in particular may be of interest to the general fzf-vim community: `VimRuntime` lists all loaded vim scripts and allows you to open them.

```vim
" FZF for all vim runtime files
command! VimRuntime call fzf#run(fzf#wrap({
\ 'source': split(substitute(execute('scriptnames'), ' *\d*: ', '', 'g'), "\n"),
\ 'options': ['--prompt', 'Vim> ', '--nth=2'],
\ }))
```

I think this command is worth adding to the main fzf-vim distribution because it is similar in scope to the already-included `Buffers`, `Commands`, `Colors`, etc. commands.

If this command is actually to be included, ideally the `substitute` call would be removed, showing the user the script number alongside each result, but doing this requires writing a custom sink plugin which is not possible from outside of the plugin (it's basically a copy of `s:buffer_line_handler`, which makes use of many other `s:` functions).

If there is interest, I'm happy to submit a PR implementing.