Changing g:mix_format_on_save on the fly

Hi, thank you very much for the plugin, it is much faster then what I was doing before.

I have a quick question though... It is not possible to change `g:mix_format_on_save` dynamically, right? For example if I want to avoid to auto formatting an specific file while still working in a single vim session that uses auto format for other buffers, manually doing `let g:mix_forma_on_save=0` will not prevent the file to be changed by the plugin, right?

By looking at the code it seems that the `augroup` is set when the plugin is loaded and then not touched anymore...

Start by reading the plugin's setup around the `g:mix_format_on_save` option and its `augroup`, as referenced in the issue. Determine how formatting is triggered for each buffer and define done as allowing the option to be changed during a Vim session so selected files can opt out while others continue formatting.

Written by the indexing model from the issue text.