[feature] possibility to implement custom line number formatting

[Mino260806]

Suggested here #517

Now configurable via Gutter.setLineNumberFormatter, by implementing LineNumberFormatter interface
Also added an example in demo

Demo

Added under View > Line Number Format > Hindu-Arabic (default selected option is Normal)

[codecov-commenter]

Codecov Report

Merging #518 (119799d) into master (4a08790) will decrease coverage by 0.01%.
Report is 4 commits behind head on master.
The diff coverage is 65.38%.

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@@             Coverage Diff              @@
##             master     #518      +/-   ##
============================================
- Coverage     73.37%   73.37%   -0.01%     
- Complexity     6598     6604       +6     
============================================
  Files           175      175              
  Lines         29867    29888      +21     
  Branches       3882     3883       +1     
============================================
+ Hits          21916    21930      +14     
- Misses         6120     6127       +7     
  Partials       1831     1831              

Files Changed	Coverage Δ
...ea/src/main/java/org/fife/ui/rtextarea/Gutter.java	82.33% <50.00%> (-0.95%)	⬇️
...ain/java/org/fife/ui/rtextarea/LineNumberList.java	62.54% <72.22%> (+0.30%)	⬆️

... and 1 file with indirect coverage changes

[bobbylight]

Thanks for the PR!