Add option to disallow intra word emphases only for underlines.

[NEricN]

This solves an issue where asterisks are desired, but not underlines. This is a common feature in a few engines, and noted by the CommonMark spec.

[codecov-io]

Codecov Report

Merging #48 into master will increase coverage by 15.79%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           master      #48       +/-   ##
===========================================
+ Coverage   61.83%   77.62%   +15.79%     
===========================================
  Files          14       14               
  Lines        3448     3795      +347     
  Branches     1056     1056               
===========================================
+ Hits         2132     2946      +814     
- Misses        788      798       +10     
+ Partials      528       51      -477

Impacted Files	Coverage Δ
bin/hoedown.c	55.94% <ø> (+13.21%)	:arrow_up:
src/document.c	87.74% <100%> (+17%)	:arrow_up:
src/escape.c	94.11% <0%> (+2.94%)	:arrow_up:
src/html_blocks.c	49.23% <0%> (+3.07%)	:arrow_up:
src/html5_blocks.c	25.28% <0%> (+4.59%)	:arrow_up:
src/buffer.c	57.74% <0%> (+7.74%)	:arrow_up:
src/context_test.c	98.31% <0%> (+8.4%)	:arrow_up:
src/stack.c	68.75% <0%> (+9.37%)	:arrow_up:
src/autolink.c	42.7% <0%> (+15.62%)	:arrow_up:
... and 4 more

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[kjdev]

Thanks.