Add features that indicate offsets of feature-based matches

[johann-petrak]

When matching against features, at the moment the begin offset of a new annotation will always be the begin offset of the annotation matched, even if the match occurs at an offset > 0 in the feature string. We should at least add features that indicate which offsets inside the actual match we got, and maybe also the actual string we matched against and the part we matched. That way the match annotations can get adapted by postprocessing, if necessary.
Example: we match the root but we match within the word and want to identify any prefix.