[pig] Support parameter substitution preprocessor statements

%default should be proposed in popup but not set if empty.

%declare should not be listed as a parameter

%declare CMD `$mycmd $date`
A = load '/data/mydata/$CMD';
B = filter A by $0>'5';

http://pig.apache.org/docs/r0.13.0/cont.html#Parameter-Sub

https://groups.google.com/a/cloudera.org/forum/#!topic/hue-user/RPk3gqNQtBo