Add support to write BrainVision CapTrak files

[sappelhoff]

Writing BVCT files could be a nice feature when somebody wants to just write montage info, and not an entire dataset (which is what #44 would imply).

The API should be simple. It could potentially support accepting an mne.DigMontage object, and/or just numpy arrays / lists / dicts.

• xref: Allow for roundtrip between DigMontage.save() and mne.channels.read_dig_fif  mne-tools/mne-python#12994