Provide Atom commands to control analysis_server

[austincummings]

As it is now, the analysis_server will keep running even after leaving the context of your Dart project. This could be fixed by just providing a command to shut down the analysis_server. Could also be useful to provide a startup command as well.

[radicaled]

The analysis_server only uses about 200MB per instance on my machine, so I don't consider this a high priority. Tagged with [help wanted] in case someone else wants to deliver.