Add version check

[twpayne]

As chezmoi gathers features, source directories might depend on features that are not present in older versions of chezmoi. chezmoi should include a way to specify the minimum version of chezmoi for the source directory to apply correctly, and issue an error or warning when the current version of chezmoi is too old.

[kkirsche]

Do you envision this being granular, at a file or directory level or per feature / capability, or more coarse like a general this version or newer style check?

[twpayne]

I think a general "minimum chezmoi version" field in the source state somewhere (maybe in a .chezmoiversion file) is sufficient.