Runscript 1.0 Beta 4

I have stopped calling it 2.0.

Changes since last beta:

• New -s option lets you run shell scripts with the runscript shell.
• Old parser now behind a feature flag, and automatically applies when you run an old script
• New options: ::default-script and ::default-phase, which allow you to manually specify the defaults.
• ::default-phase lets you run multiple phases as the default (e.g. build and run)
• Lots of general code cleanup