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

Todo:

- [ ] allow auto separately for mutliple sensors
- [ ] show basic sensor examples / update existing examples
- [ ] consolidate API including default values
- [ ] deal with projs
I like the idea of an example that estimates the cov and demos the quality
plots.

but to me it should be short to avoid scaring average users as it's really
fundamental