Not ready for a PR, but maybe as suggestions

[plan44]

I am using cebe-markdown for rendering my firmware release notes for quite a while now. For that, I had to add a few enhancements, and intended to make PRs out of it. I even prepared branches for that back then (2022) but these are not up to the contribution guidelines, so I never posted them.

The additions are:

• trait for non-standard inline colors:

  _(red)this is displayed red_, _(#0F0)this is green_
  

  This addition in particular was also inspired by this discussion about lacking color support in github markdown.
• trait for rendering _ as underscore (instead of alias for emphasis)
• option to allow prefixing URL with + to make the link open with <target="_blank">
• p44markdown class with the option for variable substitution and conditionals

All of this is visible on my fork, just in case it might be of use for somebody else using cebe-markdown. As the repo seems dormant since 2020 I do not expect a lot of interest, but also I wanted at least to make this available in some form.

[Neustradamus]

@plan44: Thanks for this ticket ^^