Changes the TAG and ATTR patterns to be more in line with HTML5 (#1297)

[dpvc]

This PR makes the parsing of tags and attributes more compliant with he HTML5 spec, which has much looser parameters for tag and attribute names. XML parsing is more restrictive, but since the parser must work for both (it is not very sophisticated), it needs to use the less restrictive version (HTML5).

The main change is that attribute names don't have to start with a letter. The other restrictions remove the control characters and the other disallowed characters. See #1297 for some discussion of the specs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.71%. Comparing base (4d1e195) to head (a6751d5).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1302     +/-   ##
==========================================
  Coverage    86.71%   86.71%             
==========================================
  Files          337      337             
  Lines        83979    83980      +1     
  Branches      4750     3124   -1626     
==========================================
+ Hits         72826    72827      +1     
- Misses       11130    11153     +23     
+ Partials        23        0     -23     

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[zorkow]

lgtm.