ENH: add helpers to merge extra data into a cycler instance

[tacaswell]

From private discussions with @weathergod

This still needs:

• is this actually a good idea to include?
• tests
• make sure it is in the docs + example

[codecov-io]

Codecov Report

Merging #47 into master will decrease coverage by 1.58%. The diff coverage is 40%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage     100%   98.41%   -1.59%     
==========================================
  Files           1        1              
  Lines         184      189       +5     
  Branches       51       53       +2     
==========================================
+ Hits          184      186       +2     
- Misses          0        3       +3

Impacted Files	Coverage Δ
cycler.py	98.41% <40%> (-1.59%)	:arrow_down:

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[tacaswell]

@WeatherGod took 18 months, but I addressed your comments!

[timhoffm]

Is there a reason these are stand alone functions? I would have expected

• a class method Cycler.from_dicts
• an instance method Cycler.merge_supplemental

[tacaswell]

@timhoffm Fair, but I think that one level of nesting cycler.Cycler.from_dicts and neither function needs access to the class object.

Still not sure this is a good idea. Maybe this should just go into the docs as an example?