MRG: 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
I don't think this can be a string, can it? I thought `str` was only used for `cuda`, which doesn't apply here (and should throw an error if someone used it).