Add method for sharing "config" values defined in COND files

[geoffxy]

Initial proposal

• Inside COND files, one can "include" other files using include("//path/to/file.cond") syntax.
• The string passed to an include() is a "path" to the file to include. The file can be located anywhere in the project, but must end in .cond.
  • The path can be "absolute" in terms of the project root (e.g., "//path/to/file.cond")
  • The path can also be relative to the current COND file's location (e.g., "../file.cond")
• Included files are interpreted as Python programs.
• Included files cannot define tasks.
• Included files cannot include other files.

The restrictions in the last two points are aimed at keeping includes and COND files simple to reason about.