Added ability to add separators as a row inside the table.

[eblis]

Added ability to add separators as a row inside the table.
Separators can be inserted by adding a single element list containing the SEPARATOR text; whenever a SEPARATOR is encountered a horizontal line will be written instead.

[codecov-io]

Codecov Report

Merging #51 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   99.57%   99.57%   +<.01%     
==========================================
  Files           8        8              
  Lines         466      471       +5     
  Branches       76       78       +2     
==========================================
+ Hits          464      469       +5     
  Misses          1        1              
  Partials        1        1

Impacted Files	Coverage Δ
terminaltables/width_and_alignment.py	100% <100%> (ø)	⬆️
terminaltables/base_table.py	100% <100%> (ø)	⬆️
tests/test_all_tables_e2e/__init__.py	100% <0%> (ø)	⬆️

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