Improve pcolor methods to plot Paerson's correlation matrices and DataFrames

[joa-quim]

Let the annotation angle be passable via an extra member of xticks or xaxis=(custom=(...))

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.97%. Comparing base (ffab0c7) to head (c70dbc0).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/pcolor.jl	95.91%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1682      +/-   ##
==========================================
+ Coverage   85.88%   85.97%   +0.08%     
==========================================
  Files         160      160              
  Lines       22511    22546      +35     
==========================================
+ Hits        19334    19384      +50     
+ Misses       3177     3162      -15     

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