Add FileExtension TextMode plugin

I wrote a component, which gives the possibility to change/add/remove the join of a file extension and the text mode.

Also a API for adding more extensions by plugins is added to the editor.

Refer to this discussion: https://github.com/Codiad/Codiad/issues/195

PS: The component could be run as a plugin too, but it doesn't make sense for me, to use a Codiad without it, because without it, there will be no extension joins in the editor.