Allow notebook authors to disable lint checks other than by comments in code?

[ceball]

This could be done by any or all of cell tags, cell metadata, notebook metadata.

And when running nbcelltests.lint, you should be able to define how notebook authors can disable checks (if at all).

[ceball]

Disabling via comment was added in 0.2:

I'm not sure I like disabling rules such as number of cells per notebook via a comment in a code cell, but this might allow someone to integrate with a pre-existing system that already uses such comments for python code.

[timkpaine]

duplicate, moving to #171