Shortened code lines (partly using yapf), wrapped comments

[deeplook]

This PR mainly wraps comment lines manually, shortens code lines and applies some readability improvements mostly as suggested by yapf. This helps generate a PDF version with index cards like in my fork. But as usual in such cases, the result is sometimes a bit of a compromise.

[rougier]

Looks good to me.

[deeplook]

This is the respective PDF output with appropriate fonts: https://github.com/deeplook/numpy-100/blob/master/100%20Numpy%20exercises.pdf

I feel like the MD parser needs to be more general to accommodate for cards showing arbitrary sequences of paragraphs and code, like in Jupyter notebooks. In fact, I had a go at this before with ReST and dedicated directives. I might give it another go. Would you be open to switching from MD to ReST?

[rougier]

Not really but we can have a rst version alongside the md version.