0.5.7 MVP New feature: Install beta themes

New:

• Install a theme from a Github repository for testing. Thank you to @ankushg @eleanorkonik for suggestion issue
• Update beta themes installed. This does not work with community themes, just themes registered with BRAT
• Install a theme from the community (these are not registered with BRAT, this is just for convenience to quickly add a community theme)
• Settings: delete beta themes from the BRAT list of themes

Improvements:

• Reorganized the README content since the feature set has expanded so much