VIZ, ENH: Add event dict to epochs plot signature

[drammock]

as discussed in #7111, this adds an event_id parameter to epochs.plot() so that if users are passing in events that aren't part of epochs.events, they can specify event colors by name instead of by integer.

It also allows user-specified event colors for the t=0 event to override the (default) transparent green vertical line at t=0. On this PR:

catch_trials_and_buttonpresses = mne.pick_events(events, include=[5, 32])
epochs['face'].plot(events=catch_trials_and_buttonpresses, event_id=event_dict,
                    event_colors=dict(buttonpress='red', face='blue'))

yields this:

If no events are specified, or if the events passed to plot() don't include the event at t=0, the default line is still drawn (though it's been moved to zorder=0 to lie behind the traces instead of on top of them):

buttonpresses_only = mne.pick_events(events, include=[32])
epochs['face'].plot(events=buttonpresses_only, event_id=event_dict,
                    event_colors=dict(buttonpress='red'))

[agramfort]

+1 for MRG provided CIs are green. Add a what's new entry before merging plz

thx

[codecov]

Codecov Report

Merging #7126 into master will increase coverage by 0.25%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #7126      +/-   ##
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+ Coverage   89.48%   89.73%   +0.25%     
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  Files         444      444              
  Lines       79002    79138     +136     
  Branches    12678    12692      +14     
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+ Hits        70696    71017     +321     
+ Misses       5456     5314     -142     
+ Partials     2850     2807      -43