Plugin Manager (CLI)

This addresses #455 in a command-line fashion (laying the foundation for doing it in the web interface, eventually -- and with your guys help, without requiring a restart of strider [or at least having it self-restart])

Later I wish to add `install` and `uninstall` but I want to pause here and submit the PR to get some feedback before going any further.

See related project https://github.com/Strider-CD/strider-plugins for where "list remote" is getting its data.

Please note an ulterior motive here to move away from npm-based plugin management and towards a git-oriented one where the `strider-plugins` project maintains a canon for plugin and their stability, compatibility, and whatever other metadata in an easy to contribute way (npm permissions are a bitch, and there are too many strider-* projects on npm with no source code links, etc... i want to stop that and keep it 100% open. too many issues with people doing npm-update and being told to blow away their node-modules durectiry.... id rather we use git end to end for all plugin management.).

Looking forward to feedback and guidance
Hmm, I didn't create any globals. I think you're referring to pre-existing code. I don't really know what that config object is doing but it seemed like the right place to grab the extensions path from. I was also confused by the fact that the extensions path comes in array-form -- as though you can have extensions in multiple places.