Add update method to Text

[thallada]

Sorry to overwhelm you with yet another PR. I'm playing around with trying to make markovify models out of huge corpus and I keep finding ways to eke out a little more saved memory.

This is mainly useful for incrementally building one Text from separate chunks of a corpus too large to store in memory all at once. This is slightly better than using combine() which requires you to have the overhead of at least two models in memory at once.

Chain now accepts base_model which the new Text update method uses.

I also tweaked how Text/Chain behave when you initialize them with None or '' so that you can initialize an empty Text without it throwing an exception and then later call update() on it.

[coveralls]

Coverage remained the same at 100.0% when pulling 696e544 on thallada:model-update into f08065c on jsvine:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 00c51bf on thallada:model-update into f08065c on jsvine:master.

[jsvine]

Thanks for suggesting this, @thallada! The goal makes sense. One question, though: What's the use case in self.build for a base_model that isn't self.model?

Put another way: What would you think about converting def build(self, corpus, state_size, base_model=None) to def build(self, corpus, state_size, overwrite=False)? If you wanted to add to the pre-existing model, you'd pass overwrite=True.

But perhaps I'm glossing over something important. What do you think?

[coveralls]

Coverage remained the same at 100.0% when pulling 806348c on thallada:model-update into c41af53 on jsvine:master.

[thallada]

I finally got around to addressing your comment. I think you are right that there is really no use for base_model and that it could be simplified with just an overwrite boolean parameter.

I made those changes and rebased with master. But, the 2.7 tests are failing in Travis and throws an error on showing me the log... I ran the tests on 2.7 on my machine and they all pass so I'm not sure what's up with that.

[thallada]

After ~4 years I've forgotten most of the context for this PR and it now has conflicts. I'm no longer working with markovify so I don't think I have the time/motivation to clean this up and test it again. Let me know if this is useful to anyone else and maybe I can help implement it again.