Support for user components directories

[keianhzo]

This PR adds support for custom user components directories.

The user can add multiple directories and the components found in those directories will be loaded at add-on load time.

We don't do hot reloading as it can get quite complex quickly and Blender already requires reload for it's own scripts paths AFAIK.

Required by https://github.com/MozillaReality/hubs-postprocessing-addon
Closes #288

[Exairnous]

Thanks for this!
There were a few issues that I found that I think should be addressed, but I think they should be easy to solve. I've left inline comments for them.

I also noticed in a couple places you're using enumerate but discarding the index. Is there a reason why you aren't just looping over stuff regularly in those places?

[Exairnous]

Oh, I just realized, when add/removing directories, the preferences should be marked dirty:
context.preferences.is_dirty = True

This is for when the user has selected that preferences shouldn't auto-save.

[Exairnous]

The dirty state of the preferences isn't updating when the fields are modified, which is weird. I suppose you could add a lambda to the update argument for the string properties and manually mark them dirty, but not sure why it's not doing it automatically. Anyway, I'll leave it up to you on whether you want to support this right now or not.

LGTM. Thank you for the changes.