Loads of stuff

[patrickbkr]

• New code and directory layout
• system version that allows switching to a preinstalled Rakudo
• Registering external Rakudos
• env mode that works by modifying PATH (similar to what perlbrew does)
  • defaults to env mode on Linux
  • defaults to shim mode on Windows (Windows doesn't support env mode)
• fish, Zsh and sh support
• Autocompletion in Bash, fish and Zsh
• build now builds the latest release instead of the master branch by default
• Make sure the rakudobrew path is only ever added once to $PATH
• some more documentation, especially for the above

Tested on Linux and Windows. It's necessary to reinstall rakudobrew to upgrade to this new version. When upgrading a previous rakudobrew to this new version rakudobrew will show a message telling the user to reinstall rakudobrew and how.

[patrickbkr]

I just implemented downgrading to version 1. And added info messages on how to do that. There is also a new branch v1. There I added a message to the self-upgrade command to tell the user she is locked to version 1 and how to upgrade to v2.