Add initial support for toggling theme

Basic support for switching theme alongside color scheme.

For testing purposes, I've been using Soda Light and Dark, which seem to switch perfectly. In order to keep the scheme and theme in sync, I've add a current_mode field, which is saved in `Packages/User/SolarizedToggle.sublime-settings` to persist across sessions.

One thing to note might be to make sure users are aware that this will overwrite their color scheme and theme settings in `Packages/User/Preferences.sublime-settings`, and they should first update `Packages/User/SolarizedToggle.sublime-settings` to ensure they don't lose their settings. Both in the initial install and the upgrade note.