Refactored grid classes to put them in an inheritance hierarchy

[tleonhardt]

Refactored grid classes to put them in an inheritance hierarchy with an abstract base class at the root.

generate_table() now takes an instance of a Grid class for grid_style instead of a class type.

There is a global DEFAULT_GRID which can be overriden with the set_default_grid() method.

AlternatingRowGrid now takes two optional arguments in its initializer: bg_primary and bg_alternate for the two alternating colors.

Also:

• Added the start of a color.py example for showcasing colored output
• Updated README
• Minor formatting tweaks to the examples

[tleonhardt]

@anselor Putting the grid styles into an inheritance hierarchy and having the generate_table method and TableFormatter class accept an instance within this hierarchy seems like a good idea to me.

But please review this and make sure it doesn't present a headache or undue burden to you.

[codecov-io]

Codecov Report

Merging #13 into master will decrease coverage by 0.65%. The diff coverage is 93.98%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
- Coverage   78.94%   78.28%   -0.66%     
==========================================
  Files           1        1              
  Lines         798      783      -15     
==========================================
- Hits          630      613      -17     
- Misses        168      170       +2

Impacted Files	Coverage Δ
tableformatter.py	78.28% <93.98%> (-0.66%)	:arrow_down:

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[anselor]

Looks good to me. API changes are relatively minor.