Not ready for a PR, but maybe as suggestions

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:
```markdown
_(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](https://github.com/github/markup/issues/1440).
- trait for rendering `_` as underscore (instead of alias for emphasis)
- option to allow prefixing URL with `+` to make the link open with ``
- `p44markdown` class with the option for variable substitution and conditionals

All of this is [visible on my fork](https://github.com/plan44/cebe-markdown/tree/luz), 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.