Add tagging support

[Gozala]

We want to utilize KSP tags. More specifically we want to enable tagging relevant segments of the web page so they could be surfaced in the context of the associated tag. It seems like enhancing xcrpt extension to allow tagging captured excerpts would give us just that.

On the other hand UX of xcrpt isn't exactly well suited for quick tagging, as it brings up a whole new page etc... Just highlighting a section and tagging is more in line of what we want.