[WIP] Plugin Management: Plugin search / discovery

[bryphe]

This is a WIP to implement a plugin search / discovery experience in the plugin pane. Note that the plugin pane is currently experimental and off by default - needs to be enabled with the sidebar.plugins.enabled setting.

This adds a search bar to the plugin pane, and when a search query is typed, it will query against a set of PluginRepository's. Right now, only a VimAwesome strategy is implemented, but the plan would be to query against these places:

• VimAwesome - for vim plugins
• NPM - for Oni plugins
• marketplace.visualstudio.com - for VSCode plugins (once we have support)

Next steps after this:

• Hook up an install button so that we can actually install items from here. There's already an API for this - Oni.plugins.installer.install / uninstall - just need to wire up the UI to it!
• Hook up a notification to show when a plugin is starting install / install completed / if there were any errors
• Add a way to uninstall