Allow extended set for <num> and ignore ref style links, already linked items, and attr_list cases with '#' before the ref

[yqbear]

The <num> part of the reference can now match any letter, digit, underscore, or hyphen.

For example:

TAG-xy12_3-z  ==> [TAG-xy12_3-z](http://gh/xy12_3-z)

Any reference preceded by a #, [, or / character is now ignored. This allows ignoring of already linked references, reference style links, and basic use of the pymarkdown attr_list extension. The following are now ignored in the markdown text.

[TAG-123](http://gh/TAG-123)
[TAG-456][tag456]
[TAG-456] [tag456]
## Feature 1 { #F-001 .class-feature }

[tag456]: http://gh/TAG-456

Tests were added for each of these items. Tests and build pass.

[yqbear]

Please use if this helps and thank you for this plugin. It has been very useful for some of my documentation projects.

[chripo]

LGTM!