Add checkpoint-interval option

[scottaiton]

Adds a checkpoint-interval option that outputs a checkpoint when iframe%checkpoint-interval == 0.

[cburstedde]

Cool! Wondering in general about the best principle on interval configurations. Does it make sense to allow for a special value to deactivate the feature? Mathematically, infinity would make most sense, but it's hard to represent. We had a discussion on 0 vs. -1 vs. <= 0 vs. no disabling feature before. As long as the same convention is applied everywhere I'd personally have no strong preference.