Add support for getting filter columns for a parsed query (#21)

This works the same as the "#filter_columns" method in the pg_query Ruby
gem, and returns the table name (if present) and column name for every
column that's referenced in a JOIN or WHERE clause.

To support this functionality, the nodes() and nodes_mut() functions
gain a 4th value for each result item, indicating whether the current
node can be considered a filter column. This is intended to be used when
matching a ColumnRef node, to determine whether it is part of a WHERE/JOIN
clause, or part of something else (e.g. target list, or ORDER BY, etc).