Make impromptu-utils executable.
Only record the last exit code before printing a prompt. Limit the utils to being included once.
Add CLI subcommands.

Separates the existing CLI commands into individual subcommand files.

A subcommand can be run as follows: `impromptu my-subcommand`.

The script to run `impromptu my-subcommand` would be named `impromptu-my-subcommand`.

We look for subcommands in the following order:
1. Inside node modules installed in your Impromptu's config (at `~/.impromptu`). To register the script, the node module must declare the subcommand using the 'scripts' property in its package.json file.
2. Scripts accessible in your environment's $PATH.