Fix plugin entry when host is default.

[fennecdjay]

PLUGIN="zsh-users/zsh-syntax-highligthing"
./zpm $PLUGIN
./zpm github.com/$PLUGIN

now properly use only "zsh-users/zsh-syntax-highligthing" as ~/.zpm/.plugin_list entry.

[desyncr]

We'll have to review this functionality to properly work multi-host/host-agnostic.

There are various checks against github.com, github.com's length etc. We should target them. This is probably a huge task.

[fennecdjay]

I was thinking of having a config.h,
which would contain:

#define ZPM_DEFAULT_HOST "github.com"

in config.h.

we could then use

strlen(ZPM_DEFAULT_HOST)

---

mainly because I think there is a need for config.h since I read this

[desyncr]

Oh, yes. main source is already 413 lines. I'd like to have a lib/ directory with mostly helper functions. Having a config.h would be neat too :)

[desyncr]

I'm gonna merge this for now but before 0.1.0 we should be host agnostic.