Add generate() for Text

[alvations]

C.f. #736 and #2251

The lidstone LM is a much slower than MLE so the generate() didn't try to replicate Lidstone with 0.2 gamma but uses the MLE model, 73f7e7b#diff-f5cb9102f911e93ca5fefa00b56a1e3f

[iliakur]

Wow, that was quick! I have a lot of questions :)

[alvations]

Thanks @copper-head for the review and all the helpful suggestions! I've committed the appropriate changes =)

[iliakur]

Look at all the pesky whitespace and long lines that black deleted for us :)

Thanks for implementing the feedback, lgtm!

[stevenbird]

Thanks @alvations and @copper-head... it's great to have this functionality back finally!