Support plugins

[MarcSkovMadsen]

I would like other packages like awesome-panel, awesome-panel-extensions, panel-chemistry, pn-highcharts etc. to be able to *plug intotheawesome-panel-cli`.

Accept Criteria

• [] Concept: Other packages can plug in and
  • add their own app, notebook and component templates to pn create app, pn create notebook. and pn create component.
  • add thir own commands. For example pn awesome-panel serve.
• [] Tested
• [] Documented
• [] Showcased