feat: cater for sourcing local plugin directories

[wintermi]

• Add examples of sourcing local ZSH files via the Zap plug function to the README
• Enhance to allow the activating of local plugin directories

[mamaraddio]

For the "activating local plugin directories", the code looks fine.

The usage o this feature leads to the necessity of cloning a repo without managing it with Zap
(or creating a folder with a plugin file inside), and then pass the dir path to plug.
So, why not just passing the path to the plugin?

I just want let @ChristianChiarulli to decide for this feature, personally I think is not a tragedy to provide a full path to the plugin instead of the directory name, but if this is accepted by Chris I have no reasons to disagree.

[wintermi]

No worries, I just thought it would be a nice option to help plugin developers using Zap.

[ChristianChiarulli]

hmm I thought we already had the functionality to source local zsh files