DSL: global variables

[archetyped]

When running a command with multiple put statements, it would be very helpful to have global variables accessible via DSL expressions.

While we can load a file accessible to all put commands with the --load flag, any out-of-stream variables defined therein are reset for each put command (i.e. values do not persist across the put commands).

Currently, the only way to persist a value across multiple put commands is to create a new field containing the value for all records. This could conflict with existing fields and may also reduce performance when working with large datasets.

Global variables would allow values to persist across an entire Miller command, accessible wherever Miller's DSL is used (put, filter), without having to modify the fields of the data being evaluated.