ENH: Allow different default stim_channel

This allows users not to have to type 'stim_channel=XXX' all the time. Now users can just do:
```
mne.utils.set_config('MNE_STIM_CHANNEL', 'STI101')
```
and not have to worry about it again. (Or, equivalently, set an env var.) Works the same as other mne-python config vars such as `SUBJECTS_DIR` in this way.