Correctly load plotly and vega libraries

[philippjfr]

Previously I used some horrible code to load plotly and vega dynamically when the models are used. Not only is this horrible, it also doesn't seem to work when exported in a notebook. This PR instead uses the __javascript__ variable on the models, which loads vega and plotly js when the extension is loaded. To avoid it being loaded all the time these models are no longer imported by default, instead the corresponding Pane objects do the import dynamically. This works fine in server and commandline use cases but in the notebook the models need to be imported before the extension is run. Therefore I have added the ability to load these models via the extension, much like in holoviews. So you can now do pn.extension('vega') and it will load vega js in the notebook.

[codecov-io]

Codecov Report

Merging #309 into master will increase coverage by 0.17%.
The diff coverage is 73.46%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
+ Coverage    89.1%   89.28%   +0.17%     
==========================================
  Files          63       65       +2     
  Lines        6583     6617      +34     
==========================================
+ Hits         5866     5908      +42     
+ Misses        717      709       -8

Impacted Files	Coverage Δ
panel/models/__init__.py	100% <ø> (ø)	⬆️
setup.py	0% <0%> (ø)	⬆️
panel/tests/test_vega.py	96.61% <100%> (ø)	⬆️
panel/tests/test_plotly.py	97.46% <100%> (ø)	⬆️
panel/pane/base.py	90.8% <100%> (+0.21%)	⬆️
panel/models/vega.py	100% <100%> (ø)
panel/models/plotly.py	100% <100%> (ø)
panel/io.py	66.77% <20%> (-0.47%)	⬇️
panel/pane/plotly.py	86.95% <57.14%> (-3.52%)	⬇️
panel/pane/vega.py	78.37% <70%> (-1.93%)	⬇️
... and 6 more

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