Allow specifying directory relative to root for --start-at

[skewballfox]

Mainly for calling from scripts where the subdirectory is supplied from a config file. handling some situations not handled by shell expansion might be useful.
say the directory has this structure

MyVault/
├─ Foo/
..everything else

I think the following should work

obsidian-export --start-at Foo ~/MyVault <output-dir>

MyVault and output-dir would expand to the absolute paths via shell expansion, start-at wouldn't.

I can open a PR if you are okay with adding support for this.