Highlight document contents with hit number attribute

[jan-niestadt]

(see INL/corpus-frontend#527)

/docs/DOCID/contents should add a hit number to <hl> tags, which will allow the frontend to deal with overlapping hits and hits that e.g. span across <s> tags.

<s><hl n="1">The fox <hl n="2">jumps</hl></hl><hl n="2"> over the dog.</hl></s>

[jan-niestadt]

Start (and end) position would also be useful; frontend would use this to jump to a specific hit in a fragment, for example (@KCMertens in INL/corpus-frontend#527).