More flexibility in setting anchor strength (fixes #16)

[GuyAglionby]

This attempts to allow setting anchor strengths on a per-topic basis, and within each topic on a per-word basis. I don't have a real theoretical understanding of this, but #16 suggests that it is possible and I think this method seems intuitive. Let me know any thoughts! Thanks

[ryanjgallagher]

Thanks for your patience on this @GuyAglionby and thank you for taking a shot at this update!

I've been having a bit of a hard time making time to take a look at this, though I've had it on my to do list. @gregversteeg do you have the capacity to review this pull request for allowing the anchor strength to be set on individual words?