Runscript 1.2.0

This "stabilises" rc.1, even though there hasn't been much testing anyway.

Full changelog of this version:

• run checks parent directories for Runfiles when resolving targets.
• Runscripts can include Runfiles.
• Error formatting made more shell-like
• Nonexistent positional arguments evaluate to "" instead of panicking

The attached binaries are not installers!
Simply decompress them, and they (should) work. Put them on your $PATH if you want.

I cannot provide a Windows build for this release due to a linker error when cross-compiling.