Add no_color and force_color parameters to override env vars

[hugovk]

Fixes #33, #28 (comment), #2 (comment).

https://no-color.org/ says (command-line) arguments should override the environment variables:

2. How should configuration files and command-line arguments be processed in the presence of $NO_COLOR?

User-level configuration files and per-instance command-line arguments should override $NO_COLOR. A user should be able to export $NO_COLOR in their shell configuration file as a default, but configure a specific program in its configuration file to specifically enable color.

This also means that software that can add color but doesn’t by default does not need to care about $NO_COLOR, because it will only ever be adding color when instructed to do so (as it should be).

Add both no_color and force_color parameters.

The no_color parameter takes priority over force_color, and both parameters take priority over the environment variables.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (35a885d) compared to base (fd27ce2).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #38   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          105       115   +10     
=========================================
+ Hits           105       115   +10     

Flag	Coverage Δ
macos-latest	100.00% <100.00%> (ø)
ubuntu-latest	100.00% <100.00%> (ø)
windows-latest	100.00% <100.00%> (ø)

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Impacted Files	Coverage Δ
src/termcolor/termcolor.py	100.00% <100.00%> (ø)
tests/test_termcolor.py	100.00% <100.00%> (ø)

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