Add configuration option for mapping username/hostname per-host

[ELLIOTTCABLE]

This PR implements a configuration-option that allows users to provide a mapping of replacement user- and host-names for systems they do not control, but often SSH into.

As an example, we use nspawn at my organization, and I do not have control over my username — this PR allows me to replace the infinitely-annoying elliott.cable@spawnbox-febox3-elliottcable with ec@nspawn3:

typeset -gA PURE_HOST_MAP
PURE_HOST_MAP[spawnbox-febox3-elliottcable]="elliott.cable:ec@nspawn3"

As for the rest, well, this is best understood from the documentation I've added to the README.

(Implementing this in a sufficiently-general way ended up being mildly messy; enough so that I'll understand if you don't wish to upstream this improvement. If not, I intend to main this feature in my fork; could you add ELLIOTTCABLE/pure to the list of maintained forks?)

This implements #488, and solves #614 and #136.

[sindresorhus]

Thanks for the pull request. However, I don't think this is something we want to maintain. What we could consider is exposing some kind of minimal hook to let you implement this yourself.

[ELLIOTTCABLE]

However, I don't think this is something we want to maintain.
good eyes, and a reassuring response; I like slim.

any ideas of the best approach? i'm limited on time, but can take a brief glance if it's approachable to an outsider.