Use the 'flexContext' option for nicer fuzzy anchoring

* Updated dom-text-matcher to c93abc20
* Updated Annotator to 41b88f9f (maintenance branch)

This version of Annotator uses a new method for adjusting the results of the two-phase fuzzy matching algorythm (used for anchoring). For details, see here: https://github.com/hypothesis/dom-text-matcher/commit/c93abc20bedd4450fd4eefe7d7d487aba324a088

Fixes #681.