WIP: add option for automated regularization to mne.cov.compute_covariance

Todo:

- [x] show basic sensor examples / update existing examples
- [x] consolidate API including default values
- [x] deal with projs
We definitely need to do some reorganization, maybe simple/advanced is good enough for now. +1 for opening an issue and not doing another release until it's done (but probably not this PR) :)