Change select text option to download chat
General fixes/cleanup

- Add some documentation to PluginManager
- Remove unused console logging
Add documentation and do more cleanup

- Rename _selectChannelElem() to _makeChannelActive() to be more
  accurate in naming
- Added function description comments for many of the internal functions
- Replaced a use of alert() with console.error() when dealing with an
  invalid result from the server
Add ability to enable/disable plugins (internal)