Added ability to choose only monospaced fonts

[goriy]

If 'only_fixed' parameter to FontChooser class constructor is set to
True then fonts families list is populated only with monospaced font
families.

Font list is filtered by font metric 'fixed'.

[codecov]

Codecov Report

Merging #5 into master will increase coverage by 1.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   83.48%   84.54%   +1.06%     
==========================================
  Files           2        2              
  Lines         442      453      +11     
==========================================
+ Hits          369      383      +14     
+ Misses         73       70       -3

Impacted Files	Coverage Δ
test.py	95.83% <100%> (+0.1%)	⬆️
tkfontchooser.py	80.48% <100%> (+1.4%)	⬆️

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