Adding cached rake autocomplete

[stigi]

Rake autocomplete is kinda slow.
This script caches the output for later usage and significantly speeds it up.

It generates a file .rake_tasks in parallel to the Rakefile. It also checks the file modification dates to see if it needs to regenerate the cache file.

[robb]

I'll look into writing something based on the zsh completion caching instead

[stigi]

👍 🥚