ENH: Add a cycler-specific chain function.

[danielballan]

The discussion in #1 introduced a concat method. For combining more than two cyclers, it would be convenient to assign an infix operator for concatenation, but there is no obvious candidate --- as has already been discussed.

What about a top-level chain method, akin to itertools.chain, but one that fails immediately if keys don't match? Is this useful bloat or a fair solution for multiple concatenation, laking a suitable operator?

I'm happy to see this tossed out if the answer is, "sorry, this is bloat." But I'll leave it here for discussion. If people like it, I'll add tests.

[tacaswell]

I am 30/70 on this. More open to changing the concat function to take 2 or more Cyclers.

I like the analogy, but the same-named-but-special top level function floating around can get confusing.