Added UnicodeSingleTable that uses UTF-8 characters for borders

[eblis]

Works on both Linux and Windows systems.
Also made sure the files use UTF-8 encoding.

[codecov-io]

Codecov Report

Merging #50 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #50      +/-   ##
=========================================
+ Coverage   99.57%   99.6%   +0.03%     
=========================================
  Files           8       8              
  Lines         466     509      +43     
  Branches       76      76              
=========================================
+ Hits          464     507      +43     
  Misses          1       1              
  Partials        1       1

Impacted Files	Coverage Δ
terminaltables/ascii_table.py	100% <ø> (ø)	⬆️
terminaltables/base_table.py	100% <ø> (ø)	⬆️
terminaltables/github_table.py	100% <ø> (ø)	⬆️
terminaltables/width_and_alignment.py	100% <ø> (ø)	⬆️
terminaltables/terminal_io.py	96.07% <ø> (+0.07%)	⬆️
terminaltables/build.py	100% <ø> (ø)	⬆️
terminaltables/other_tables.py	100% <100%> (ø)	⬆️
tests/test_all_tables_e2e/__init__.py	100% <0%> (ø)	⬆️

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