ENH: first pass at html output

[klauer]

(This is part of a larger branch which got to be unmanageably large - breaking it up into separate PRs)

A first pass at allowing blark to output syntax-highlighted (or HTML entity-annotated) code. I anticipated using this in documentation at some point, rather than writing something for Pygments

Things to do, maybe or maybe not in this PR:

• Remove the hardcoded stylesheet in the output
• Add test suite case for this output (done, in a smoke tested way)
• Other things I'm forgetting here (done, because I haven't recalled what these were)

A couple interesting notes:

• Stylesheet-wise: if I can make a decent looking theme for the output, I may consider adding some sample CSS as part of the package (but not in each output).
• This currently only works to syntax highlight (well, add tags with defined HTML classes - the coloring is up to you) the original code. That is to say, it won't reformat the output and then syntax highlight it. blark can't correlate its output (post-generation) to its input (the raw source code), so it'd actually require a few steps to do this. Parse code -> reformat it -> re-parse the formatted code -> add HTML annotations there.
• Anyone with an interest - suggestions are welcome as per usual. Consider this a WIP of a preliminary feature at best

Sound like a couple problems for another day...

[codecov-commenter]

Codecov Report

Merging #88 (4cd1d06) into master (a39b3f1) will increase coverage by 0.4%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master     #88     +/-   ##
========================================
+ Coverage    81.1%   81.5%   +0.4%     
========================================
  Files          26      27      +1     
  Lines        5743    5819     +76     
========================================
+ Hits         4662    4747     +85     
+ Misses       1081    1072      -9     

Files	Coverage Δ
blark/__init__.py	100.0% <100.0%> (ø)
blark/html.py	100.0% <100.0%> (ø)
blark/tests/test_cli.py	91.5% <ø> (ø)

... and 1 file with indirect coverage changes