Output files should be allowed to reference config variables

[jhclark]

The following should succeed, but instead gives a cryptic stack trace:

task t > file oops_param=@ {}
global {
  oops_param=42
}

[jhclark]

This was originally discovered when Austin forgot to put a :: before oops_param. However, we should allow users to shoot themselves in the foot in this way.

[jhclark]

There's now a test case for this in syntax/issues/shouldSuceed/140.tape