Setters for Summarizer

This addresses issue #16.

I'm also wondering if there is some correlation between topKeywords size and summary size? I.e. maybe I should have a formula that changes topKeywords size automatically depending on the desired summary size?

Also, @MojoJolo, have you seen my question [here](https://github.com/MojoJolo/textteaser/commit/82dd2d092608e1a063d9539df335abf6d4b76f98#diff-429affd673e4182f49b6fb506e38c122L97)? Can you somehow comment on that?