Add a possibility to specify options for ag-subdir and specify toplevel variables and procedures

[ethouris]

Similarly to ag-export, which transfers the variable or procedure into the makefile, add a possibility to create a variable that will be passed to the slave interpreters.

Something like ag-global, which makes the given variable (the check should enforce this variable to be global or otherwise only in a namespace-local scope) or procedure accessible inside every slave interpreter created in the subdirectory.