Add support for installing from path

[Xenira]

For testing purposes it would be great if I could install a local extension without having to go over the registry.

e.g pie install --path .

[Xenira]

Let me know if this is something you'd want and how the cli should look like. I could implement this.

[Xenira]

@asgrim Think this is duplicate of #79. Please reopen, if cli option is still something worth considering.

[asgrim]

It is something only subtly different; I need to have a think if 1) it is useful, and 2) how it makes sense for usage. Leave it with me :) thanks!