Add support for displaying callback errors and stdout in the notebook

[philippjfr]

This finally allows displaying both errors and stdout output (e.g. print statements) triggered by a callback in the notebook. This should make debugging a whole log easier. Currently this can be configured using pn.config.debug which is also settable using the pn.extension and an environment variable. By default it accumulates all the output but you can also choose to replace the output each time or display no output at all.

Requires pyviz_comms>=0.7.3 to work.

[codecov]

Codecov Report

Merging #977 into master will increase coverage by 0.09%.
The diff coverage is 91.22%.

@@            Coverage Diff             @@
##           master     #977      +/-   ##
==========================================
+ Coverage   85.74%   85.83%   +0.09%     
==========================================
  Files          98       99       +1     
  Lines       11077    11235     +158     
==========================================
+ Hits         9498     9644     +146     
- Misses       1579     1591      +12

Impacted Files	Coverage Δ
setup.py	0% <ø> (ø)	⬆️
panel/tests/test_config.py	100% <100%> (ø)
panel/pane/markup.py	97.05% <100%> (+1.1%)	⬆️
panel/util.py	90.57% <60%> (-1.22%)	⬇️
panel/config.py	53.08% <75%> (+1.11%)	⬆️
panel/io/state.py	93.18% <75%> (-1.95%)	⬇️
panel/viewable.py	81.45% <80%> (-0.08%)	⬇️
panel/tests/conftest.py	96.1% <90.47%> (-2.12%)	⬇️

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