`rewrite`: add command to rewrite tree with `<SNAP>:<PATH>` syntax

[aawsome]

features:

• allow to rewrite tree with <SNAP>:<PATH> syntax
• Add option --as-path to use with rewrite command

Implementation idea:

• read snapshot/path
• get the iterator over nodes
• apply globs/path rewrites
• call archiver to save modified trees
• save new snapshot if it is modified

[LeoniePhiline]

Maybe call it the rewrite-it-in-rust command. =)

Sorry, couldn’t resist :) 🦀

[ibash]

It'd be great if we could apply an ignore file to a snapshot. e.g. I'll oftentimes come across files that got backed up that I didn't want, I'll add them to an ignore file and would like to apply that retroactively.