Allow ignoring certain paths

[tmayoff]

I've been trying to find a problem matcher that can ignore problems with certain paths (e.g subprojects, external, etc..). This seems to be the only problem matcher that's using variables in the regex, would it be possible to add this functionality. I'd submit a PR myself, but I'm not sure on the regex, been messing around but can't get it to work properly

[tmayoff]

I made a custom problem matcher for myself, comment with the update is here. I can make a PR at some point to add the functionality here as well