Little bug with themes - fix for #4

There's an open issue #4 about a little bug with themes. In two words, the problem is when I use a theme but put the view only in the protected/views, not the theme views dir, the extension generates an error.

The [solution](https://github.com/yiiext/twig-renderer/issues/4#issuecomment-4709961) was suggested, so I implemented, improved and tested it. Hope it'll be useful.

I also added a Composer installation workflow to the READMEs, so that nobody wastes their time finding out how to do it.