[Feature Request] Automatically detect installed themes

[baroldgene]

This is a great plugin. One thing I find mildly annoying is that (unless I'm missing something) I have to define my themes in other files to install them, then list them again in Themery. While I appreciate the ability to customize what shows up in the picker, it would be convenient to have Themery automatically detect installed themes so that I only have to define them once.

Ideally this could allow some sort of filter to exclude some installed themes from the picker.