license_cpp: add ability a matcher to extract text

We need a way for a matcher to specify that a certain region of a LICENSE isn't relevant and extract it.

## input
```
License: foobar
----
license for blah.txt

Not relevant license
```

## output
```
License: foobar
```

## example
https://chromium.googlesource.com/chromium/deps/icu/+/refs/heads/main/LICENSE#469