provide a --backup parameter for apply cmd

[parisni]

Is your feature request related to a problem? Please describe.

I was in the process of migrating from no dotfile manager to chezmoi on two laptops.

1. I used ch add files on first laptop
2. I cloned the config on second laptop and templated the files to handle the config diff
3. The diff became very difficult to read and I could not apply with 100% trust, so I did backup existing files on second laptop (added .back on every files managed by chezmoi) and then applied
4. Turns out I broke some files (lost few lines) then I was happy to have the backup

I would find useful to provide a built-in --backup parameter to let chezmoi do itself the backup for every single file it would overwrite.

WDYT ?

EDIT: used that command to dump files before apply

chd |awk '/diff --/{gsub(/a\//, "", $3);gsub(/.*/,"cp " $3 " " $3 ".back",$3);print $3;system($3)}'