Fixes for Markdown CSS

[philippjfr]

This overrides some of the global bokeh CSS properties for the Markdown pane ensuring appropriate padding and margins for tables and lists and improved styling for tables.

Fixes #323

[codecov-io]

Codecov Report

Merging #328 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
+ Coverage   89.53%   89.54%   +0.01%     
==========================================
  Files          84       84              
  Lines        6784     6792       +8     
==========================================
+ Hits         6074     6082       +8     
  Misses        710      710

Impacted Files	Coverage Δ
panel/pane/markup.py	93.75% <100%> (+0.69%)	⬆️
panel/tests/test_interact.py	100% <100%> (ø)	⬆️
panel/viewable.py	83.83% <100%> (ø)	⬆️
panel/pane/tests/test_markup.py	100% <100%> (ø)	⬆️

---

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[codecov-io]

Codecov Report

Merging #328 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
+ Coverage   89.53%   89.54%   +0.01%     
==========================================
  Files          84       84              
  Lines        6784     6792       +8     
==========================================
+ Hits         6074     6082       +8     
  Misses        710      710

Impacted Files	Coverage Δ
panel/pane/markup.py	93.75% <100%> (+0.69%)	⬆️
panel/tests/test_interact.py	100% <100%> (ø)	⬆️
panel/viewable.py	83.83% <100%> (ø)	⬆️
panel/pane/tests/test_markup.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f90d5c2...8369937. Read the comment docs.