Consider defaulting `toast incremental` to the current directory

[johno]

This is more a general design consideration than anything, it's not a hill I'd die on. But, in my opinion, it makes sense to default to the current directory for the toast incremental command. It's sort of implied and then one doesn't have to specify the ..

This does introduce a bit of magic, perhaps, and attempting to build the current directory could trip up new users. Though, since site building is a subcommand rather than top-level I don't think that it'd be too surprising.

[ChristopherBiscardi]

yeah I think we can default to this since it's a subcommand.