Add min_words optional argument to make_sentence

[brienna]

Lets us specify the minimum number of words we want in the sentence.

Complements the max_words optional argument.

Example:

model.make_sentence(min_words=10)

[coveralls]

Coverage remained the same at 100.0% when pulling a43ad84 on briennakh:master into 3f3bfd9 on jsvine:master.

[jsvine]

Hi @briennakh, and many thanks — this looks like a very reasonable addition! Could you add a test that covers this new parameter, so that we retain 100% code coverage? (See Coveralls check above.)

[jsvine]

Thank you, and looks good! Merged and now available in v0.8.1.