ENH: expand call signature of cycler()

[WeatherGod]

In the discussions going on over in matplotlib/matplotlib#4686, some have asked for the cycler() call signature to be expanded a bit. Now, this can be a rabbit hole, so I will break out the proposal into stages so that we can decide just how deep we want to go:

Current signature (I call this the simple positional):
cycler(key, vals)

1. Allow a single kwarg to be equivalent to the current notation:
   cycler(k=vals)
   Example:
   cycler(color='rgb') vs.
   cycler('color', 'rgb')

2. Allow for multiple kwargs (all assumed to be "added" together):
   cycler(k1=v1[, k2=v2, ...])
   Example:
   cycler(color='rgb', lw=[1, 2, 3]) vs.
   cycler('color', 'rgb') + cycler('lw', [1, 2, 3])

3. Allow for multiple positional args (all assumed to be "multiplied" together):
   cycler(k1, v1[, k2, v2, ...])
   Example:
   cycler('color', 'rgb', 'lw', [1, 2, 3])
cycler('color', 'rgb') * cycler('lw', [1, 2, 3])

For 2 & 3, I would completely disallow the mixing of positional and keyword arguments.
If we want to avoid having two different operations possible from a single cycler(), we could forgo option 3, and go for the following:
3alt) Allow for both positional and kwargs, but assume all to be "added" together:
cycler(k1, v1[, *args][, k3=v3, **kwargs])

Whichever option (if any) people want, I can put together a PR that implements that. Ideally, I would like to have cycler.cycler() to have very similar, if not identical, call signatures to my validating version that I am making in the matplotlib PR referenced above.

[efiring]

I think that including #3 makes everything too confusing and hard to remember. #1 and #2 make good sense to me, and I can see #3alt as a reasonable but not essential generalization--and probably not worth the extra complexity in implementation and documentation. I don't see any obvious use case for for which #3alt is advantageous. The use case for #2 is that it facilitates
ax.set_prop_cycle(**kw) as well as ax.set_prop_cycle(color='rgb').

[efiring]

Can the cycler() signature be modified to accept only the following two signatures?

c = cycler(pre_existing_Cycler_instance)

and

c = cycler(**kw)

In other words, eliminate the present two-argument form as completely unnecessary when the **kw form is available. I don't see any advantages to the two-argument form.

[WeatherGod]

It is still early enough in this project's lifetime to make a complete change like that. It isn't like doing an API change for matplotlib. We can always go back to including positional arguments in the future, if we want it.

[WeatherGod]

note, I could also implement it where multiple positional arguments (more than one pair) would be excluded. (actually, I already have, I just haven't submitted it yet)

[WeatherGod]

Just realized that we would need to update the repr's because they use the positional form. Not terrible, but just an additional thing to complete.