Plugin System

Add a system to support plugins: shared libraries that can be loaded dynamically to add functionality to the log viewer.

Plugins should be able to:
* Add new tab types that can be opened in a log view.
* Register hotkeys.
* Add a new log source (e.g. to download logs from S3 or CloudWatch).
* Subscribe to arbitrary events from the viewer (log file opened, log file changed, selection made.)