Set 'commentstring' option

Hello,

I just saw the plugin about half an hour ago and downloaded it to check the commenting/uncommenting lines in .gitignore files with vim-commentary plugin. But it didn't work. So I fixed that by putting the line

```vim
setlocal commentstring=#%s
```

to the syntax file. My guess is that with `'commentstring'` option set correctly you can remove `plugin/gitignore.vim` file and tComment plugin will still work as expected. But I don't have tComment installed, so I didn't touch the file.

I also noticed some very minor issues with ftdetect plugin and one of snippets and fixed them.

Thanks for the `gitignore.vim`!