feat: Add working tree config option

[twpayne]

Fixes #891.

[felipecrs]

This is interesting. So now, I should probably add a workingTree to my chezmoi config template?

[twpayne]

It should be optional. chezmoi should automatically detect the working tree, but there could well be places where it does not work as expected. For sure, if sourceDir is a subdirectory of your working tree (e.g. home, as in your case) then you need to set sourceDir in your config file, but you /shouldn't/ need to set workingTree. Please tell me if you find any unexpected behavior.

[felipecrs]

Ok, will do. Thank you!