Allow TikZ plots to append options

[isteinbrecher]

I added the keyword option append_options to the Plot class, which allows to optionally appended the given to the automatically assigned TikZ cycle list (via \addplot+, cf. pgfplots manual Section 3.3.2).

This is my first time contributing to this project, so please feel free to tell me if I missed something (eg. maybe this feature is already possible some other way) or got the workflow wrong.

[codecov-io]

Codecov Report

Merging #312 (f13c9cf) into master (71bac0b) will decrease coverage by 0.02%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
- Coverage   89.74%   89.72%   -0.03%     
==========================================
  Files          44       44              
  Lines        3834     3843       +9     
==========================================
+ Hits         3441     3448       +7     
- Misses        393      395       +2     

Impacted Files	Coverage Δ
pylatex/tikz.py	92.23% <87.50%> (-0.33%)	⬇️

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