PR for v.1.3.4

[hcpchris]

• ENHANCEMENT: Converted ButtonTheme into an extensible descendent of JavaScriptDict (Closes 'navigation' not implemented..? #86 ).

[codecov]

Codecov Report

Merging #88 (ab19eeb) into master (9ccc051) will decrease coverage by 0.06%.
Report is 1 commits behind head on master.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   90.34%   90.28%   -0.06%     
==========================================
  Files         206      206              
  Lines       28057    28135      +78     
  Branches     1929     1948      +19     
==========================================
+ Hits        25348    25403      +55     
- Misses       1985     1994       +9     
- Partials      724      738      +14     

Files Changed	Coverage Δ
highcharts_core/metaclasses.py	62.86% <52.77%> (+2.65%)	⬆️
highcharts_core/utility_classes/buttons.py	83.75% <57.40%> (-4.10%)	⬇️

... and 1 file with indirect coverage changes