Review multi-host support

[desyncr]

• Review implementation to be fully host agnostic (github, bitbucket, etc)
• Avoid hardcoding hosts (try to...)
• Add tests

@fennecdjay I have yet no idea how to implement this so any suggestion is welcome!

We should discuss here before doing any PR :)

TODO

• Using "https://" causes plugin install to fail
• It does creates "<hostname>/user/repo.git" folders

[fennecdjay]

Maybe we could have

#define ZPM_DEFAULT_HOST "github.com"

in config.h then

if (!(host = getenv(ZPM_DEFAULT_HOST)) {
    host = ZPM_DEFAULT_VALUE;
}

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I can give it a try tomorrow if you want.

[desyncr]

Done in #28