Store and reuse interim results

[PaulSzymanski]

This speeds up random walks significantly.

[coveralls]

Coverage remained the same at 100.0% when pulling 0dae4b0 on PaulSzymanski:reuse into 6968649 on jsvine:master.

[jsvine]

Thanks, @PaulSzymanski! Certainly seems worth exploring. A few questions for you and for repository-watchers:

1. I'm a bit concerned about how this change would affect memory usage, especially for very large corpora. Do you have any benchmarks to share on that?

2. Given the above, what do you think of making this feature opt-in, via a keyword argument. E.g., cache_interim_results = True, memoize = True, or somesuch?

3. Thoughts of using if state in self.mem: rather than the try/except block?

[jsvine]

Closing, due to the fact that this concept was implemented in #113. Feel free to reopen and/or comment, however, if you disagree. Thanks!