Allow setting ZPREZTODIR to make prezto easier to integrate with

[belak]

This is arguably an annoying hack, but it lets the maintainers of antigen remove a bunch of weirdness which makes sure zprezto is in a folder named .zprezto and changing the ZDOTDIR at runtime to make loading this possible.

Obviously the name of the variable can be changed to whatever.

[paulmelnikow]

Thanks for opening this.

For background, antigen is a plugin loader that (among other things) lets non-Prezto users easily load plugins from the Prezto repo. Because our module loader hard-codes .zprezto, Antigen must special case Prezto installation, and temporarily override ZDOTDIR when loading plugins. Prezto shouldn't be so fussy. This is an easy fix so that we don't have to be installed in a .zprezto folder.

[paulmelnikow]

Would be great to have some people test this.

[facastagnini]

Thanks for your contribution @belak