Allow Impromptu to be installed globally. Migrates use-impromptu into this repository.
Allow modules to register their own subcommands.

If a module exposes a script "impromptu-subcommand", it can be run by calling "impromptu subcommand".

To override Impromptu's location, set the "IMPROMPTU_DIR" environment variable (instead of both "IMPROMPTU_DIR" and "IMPROMPTU_BIN"). "IMPROMPTU_BIN" and the new "IMPROMPTU_INTERNAL_BIN" are automatically generated from the "IMPROMPTU_DIR" environment variable and cannot be overridden.
Improve global impromptu fallback prompt.
Remove "impromptu-" prefix from bin scripts that are not subcommands.
Update tagline.
Allow the fallback prompt to accept a message.
Add a method for logging errors.
Rename '.compiled' directory to 'tmp'.
Update prompt tests to use new error message.
Remove unnecessary modules from prompt tests.
Convert tests to JavaScript.