Support cwd parameter

[f-meloni]

Many people prefers using the SPM Danger configuration, because is more performing.
Having a Package.swift on your root folder can be annoying, especially now that Xcode (from 11) doesn't put on the recents list an xcproj (or xcworkspace) when there is a Package.swift in the same folder.
At the moment if you move the Package.swift in another folder Danger breaks because the path are relatives to the root folder.
Of course the paths can be changed on the Dangerfile in order to have .. or the full path, but this would not work for example for Plugins where the path is automatically taken from Danger's modified files.
This is why I thought would be nice to offer a current work directory option to make you run danger like it was in the root folder of your project even if you have it somewhere else.
I can also think this could be useful in cases like https://github.com/WeTransfer/WeTransfer-iOS-CI/tree/master/Danger-Swift where you have your CI configuration in a separated repo.

PS. this change to work requires the changes on danger/danger-js#1021
@orta can you please release a new danger-js version?

[danger-public]

	Warnings
⚠️	Sources/RunnerLib/SPMDanger.swift#L53 - Don't add a space between the method name and the parentheses. (no_space_in_method_call)

Generated by 🚫 Danger Swift against 139adcc

[orta]

Looks good overall, but it's not a feature unless it's documented ;)