Add ability to define explicit embed states

[philippjfr]

Fixes #1198

Simple example:

slider = pn.widgets.FloatSlider(name='A')

@pn.depends(slider)
def fn(value):
    return value + 1

pn.Row(slider, fn).embed(states={slider: [0.1, 0.7, 1]})

• Add docs
• Add tests

[codecov]

Codecov Report

Merging #1274 into master will decrease coverage by 0.01%.
The diff coverage is 86.48%.

@@            Coverage Diff             @@
##           master    #1274      +/-   ##
==========================================
- Coverage   87.73%   87.72%   -0.02%     
==========================================
  Files         124      124              
  Lines       13238    13323      +85     
==========================================
+ Hits        11615    11688      +73     
- Misses       1623     1635      +12     

Impacted Files	Coverage Δ
panel/io/notebook.py	58.64% <ø> (ø)
panel/io/save.py	57.33% <ø> (ø)
panel/widgets/player.py	82.75% <25.00%> (-2.96%)	⬇️
panel/widgets/select.py	92.92% <58.33%> (-1.19%)	⬇️
panel/widgets/slider.py	91.45% <70.58%> (-1.50%)	⬇️
panel/tests/io/test_embed.py	99.35% <97.05%> (-0.65%)	⬇️
panel/io/embed.py	85.99% <100.00%> (ø)
panel/viewable.py	68.69% <100.00%> (ø)
panel/widgets/base.py	81.17% <100.00%> (ø)
panel/widgets/button.py	83.33% <100.00%> (ø)
... and 1 more

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