Support only stripping outputs larger than a given size

[cjblocker]

I need to be able to strip embedded images and videos from a notebook while leaving the smaller text output (similar to #58). This PR adds a --max-size option which removes output larger than max-size bytes.

$ nbstripout --max-size 5K test1.ipynb
$ nbstripout --max-size 100 test2.ipynb
$ nbstripout --max-size 1M test3.ipynb

This easily delineates larger binary outputs from smaller textual ones.

I did not want to just filter on keys (an alternative) because a matplotlib animation video may be embedded in a text/html key, but I don't want to remove every text/html key. I'm also not sure if embedded images are always image/png in all kernels and plotting backends.

I've run some ad-hoc benchmarks and computing the size of the outputs in negligible to the overall run time when enabled (~2%-4%).

Thanks!

[kynan]

Thanks for your contribution! This is looking pretty good. Some suggested simplifications.

Please rebase and add a test as well!

[cjblocker]

@kynan Great, I have added your suggestions. Let me know if there is anything else!

[kynan]

Thanks, that's good to go now :)