Implement the --orig command (#4)

[H3mul]

Implemented the simple version of the proposal for feature #4.

This should resolve simple file conflicts while stowing by moving them to an .orig copy in the file location and then symlinking the target to the stow.

Added bonus: when using --orig with an unstow, it restores those .orig files back, so stow + unstow should leave the host filesystem untouched. This has no effect on unstow file conflicts.

I didn't need the flexibility of supplying the extension as another arg, or copying the backups to a different dir (as described in a further comment), but I'll be happy to implement if that feature has more demand!

[coveralls]

Coverage increased (+0.2%) to 86.465% when pulling 6135993 on H3mul:feature-4-orig-command into 4ef5eca on aspiers:master.

[orel1]

Thanks for this! It would be really useful to have this ability to ignore conflicts, especially for people like me who manage their dotfiles with stow :-)

however, I would have 2 remarks:

• Would it be possible to name this option --backup, instead of --orig? There are at least 2 good old unix tools (see cp and mv) that use this syntax and as the goal is the same it would probably be better to use the same naming.
• In the same idea of following the way cp and mv work, we could use the ~ suffix instead of .orig for renaming files