Expose title and other kwargs on show

[philippjfr]

Supersedes #650

[codecov]

Codecov Report

Merging #962 into master will increase coverage by 0.12%.
The diff coverage is 54.83%.

@@            Coverage Diff             @@
##           master     #962      +/-   ##
==========================================
+ Coverage   85.15%   85.27%   +0.12%     
==========================================
  Files          96       96              
  Lines       10714    10696      -18     
==========================================
- Hits         9123     9121       -2     
+ Misses       1591     1575      -16

Impacted Files	Coverage Δ
panel/io/server.py	34.54% <100%> (ø)	⬆️
panel/template.py	70.37% <100%> (+7.37%)	⬆️
panel/viewable.py	81.3% <46.15%> (+0.35%)	⬆️

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