Implement source reloading for `crepe s` (#6)

Allow `crepe s` to automatically reload source code in the development
environment when changes are detected. This will make developing Crepe
APIs much faster and users can avoid constantly restarting their rackup
servers.

Signed-off-by: David Celis <me@davidcel.is>
Merge pull request #7 from davidcelis/reloading

Implement source reloading for `crepe s` (#6)