Add panel entry point

[jbednar]

Passing along a suggestion that I think is a good one: so that Panel users won't be affected if we eventually do split out the parts of Bokeh that Panel uses, we should add an entry point for launching servers like panel serve file.py. For now that would just delegate to bokeh serve file.py, but we could then change that to whatever the underlying Bokeh infrastructure becomes, which could even be part of Panel itself.

[philippjfr]

This was implemented.