Add user_css option

[saimn]

Fix #313.

[codecov]

Codecov Report

Merging #418 (a3ee94f) into master (40a4a68) will decrease coverage by 0.00%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
- Coverage   86.40%   86.40%   -0.01%     
==========================================
  Files          22       22              
  Lines        1780     1787       +7     
==========================================
+ Hits         1538     1544       +6     
- Misses        242      243       +1     

Impacted Files	Coverage Δ
sigal/settings.py	97.87% <ø> (ø)
sigal/writer.py	84.50% <87.50%> (+0.13%)	⬆️

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