* Refactor path subcommand argument handling
DefaultArguments are no longer passed to each subcommand. Instead, each
subcommand has its own Path<xxx>Arguments. This means that it is no
longer necessary to edit every single path subcommand source file when
changing the arguments struct.
* Add new path join subcommand
Makes it easier to create new paths. It's just a wrapper around Rust's
Path.join().
