Add 'zigmod add' command to add from git, aquila and zpm

[zenith391]

This pull request unifies zpm add and aq add under a single add command like most package managers.

The pull request importantly adds the git subcommand to the add command for adding a package directly from a Git repository which fixes #17. Alongside the ease of use, this helps simplifying the documentation of libraries that are not on aquila and zpm by having to only list one command instead of a line to add manually in zig.mod file.

Finally, I am not sure of whether a command like zigmod add --zpm package should be used instead of zigmod add zpm package as I think flags could be added to the command to for example, save the package as a dev dependency using something like zigmod add --dev zpm package