Make `tuckr push` be able to use `^` prefix with root targeting dotfiles

[RaphGL]

If a dotfile needs to be stored outside of the home directory tuckr should be able to use ^ to do so correctly.

[MaddyGuthridge]

Started looking into implementing this, but am a little worried about this causing problems on Windows, since I am unsure how Tuckr handles root-targeted dotfiles there (where there is no one "root" of the file system). Should it target the root of the C drive? Or is this a UNIXy-systems-only feature?

[MaddyGuthridge]

Additional thought: could it be helpful to introduce a TUCKR_ROOT variable to allow users to specify what the "root" of their system should be?

• This would make testing root-targeted files easier
• This would resolve the issue on Windows where the system root isn't clear (we can just make it be C:\ by default and let users override it if their system is weird).

[RaphGL]

Ideally everything should be cross platform. Someone gave me a suggestion on another issue related to this one.
They asked for us to do what steam does, which is to have directories like ^c, ^d, ^e whatever to mimick c:\\, d:\\ and so on.

I also think this would solve your second comment