Fix Lists and upgrade List support to CSS3

[Sub6Resources]

The new CssBoxWidget allows us to effectively render a "marker box" on elements, especially list elements. This upgrade fixes almost all of our open list issues. It removes support for custom Widget list markers, since CSS doesn't support that (however, the same functionality could easily be re-added in a CustomRender by interested parties).

This pull request also removes the roman-numeral-specific numerus dependency in favor of the more general list_counter dependency (disclosure: package written by myself). This change in dependency allows us to expand our list style support to 54 different list-styles, including dozens of new languages.

This pull request fixes the following open and tightly-related issues:

Fixes #1100

Fixes #245

Fixes #984

Fixes #991

Fixes #1016

Fixes #1020

Fixes #1104

This pull request is a work in progress ready to go. Still to do:

• Re-add support for list-style-image.

[erickok]

Super work!

I will run on some examples, but the code looks solid.

[codecov]

Codecov Report

Base: 49.70% // Head: 51.41% // Increases project coverage by +1.71% 🎉

Coverage data is based on head (91a7e70) compared to base (cb23cf0).
Patch coverage: 79.05% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1161      +/-   ##
==========================================
+ Coverage   49.70%   51.41%   +1.71%     
==========================================
  Files          18       19       +1     
  Lines        2682     2641      -41     
==========================================
+ Hits         1333     1358      +25     
+ Misses       1349     1283      -66     

Impacted Files	Coverage Δ
lib/src/css_parser.dart	13.83% <0.00%> (+0.19%)	⬆️
lib/style.dart	85.36% <69.23%> (+0.59%)	⬆️
lib/src/css_box_widget.dart	64.28% <76.66%> (-0.19%)	⬇️
lib/html_parser.dart	83.96% <90.19%> (+10.61%)	⬆️
lib/custom_render.dart	50.96% <100.00%> (-0.62%)	⬇️
lib/src/style/marker.dart	100.00% <100.00%> (ø)
lib/src/styled_element.dart	80.21% <100.00%> (+1.06%)	⬆️

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[Sub6Resources]

@erickok If you wouldn't mind doing a quick rereview on the most recent commit (which re-adds basic list-style-image support), that would be great. I also cleaned a few things up so there isn't as much repeated code.

[erickok]

LGTM