FIX: Allow bads exclusion at proj stage

Addresses #466. Tests still pass over here.

However, I'm not sure we should actually support bad channels being rejected at this stage. A better procedure would be to exclude bad channels in the calculation of your projectors in the first place. If it's because of projectors that are automatically included in a given FIF file, it would make sense just to re-calculate them (I've only ever seen continuous SSPs in this context), excluding the bad channels. I added some text to the function comments related to this.

It might make sense to also throw a warning if any channels are actually excluded because of this. @chdinh and @agramfort WDYT?