add (markdown) emphasis escaping

[Crater44]

This feature adds the possibility of escaping italic on markdown

Before the only way to write a word with underscore (e.g. SYS_ADMIN) was placing a space after the underscore, now it's possible to escape it with a backslash so it won't trigger the italic feature.

Change

The change consist on combining the actual negative look-ahead for a space with a negative look-behind for a backslash

[joshgoebel]

Sadly, we cannot use negative look behind yet as that feature is still too newly added to Safari.

[joshgoebel]

Actually, hold that thought, perhaps it's time to change this policy: #3890

[Crater44]

@joshgoebel great so if my change can be added that would be great to me because I have it patched in one of my projects and is not the best way to have it

[joshgoebel]

No lookbehind for now as it would be a breaking change... why not just have a single (separate) rule that matches \* but then chooses not to highlight it? Would that hurt anything else?