datasource_plugin.go

- new builtin datasource plugin "Logging" (likely going to be renamed)
- plugin implements no panel ctrls yet, only ships datasource
- new models for logging data as first class citizen (aside from table
  and time_series model)
- Logs as new view for Explore
- JSON view for development

Testable only against existing logish deployment.
Then test with queries like `{job="..."} regexp`.