Add support for saving and loading embedded data from JSON

[philippjfr]

Makes it possible to export the embedded state to JSON files which can be loaded independently. Unlike the equivalent functionality in HoloViews each state is stored as a separate file which means it can be loaded only when it is needed, allowing a huge state space to be exported with a minimal file size penalty.

[jbednar]

Nice!

[codecov-io]

Codecov Report

Merging #301 into master will increase coverage by 0.34%.
The diff coverage is 87.34%.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   88.92%   89.26%   +0.34%     
==========================================
  Files          63       63              
  Lines        6331     6504     +173     
==========================================
+ Hits         5630     5806     +176     
+ Misses        701      698       -3

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
panel/widgets/input.py	97.72% <100%> (+0.03%)	⬆️
panel/widgets/select.py	89.92% <100%> (+0.11%)	⬆️
panel/tests/test_io.py	100% <100%> (ø)	⬆️
panel/models/state.py	100% <100%> (ø)	⬆️
panel/widgets/button.py	90% <66.66%> (-4.12%)	⬇️
panel/io.py	67.24% <72.46%> (-2.23%)	⬇️
panel/viewable.py	77.58% <78.12%> (+4.28%)	⬆️
panel/widgets/base.py	97.43% <87.5%> (-2.57%)	⬇️
panel/tests/fixtures.py	97.5% <90.9%> (-2.5%)	⬇️
... and 4 more

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