Add Spark Interpreter option to not import implicits

Add option zeppelin.spark.sqlAutoImport or something similar to Spark interpreter to not import implicits and functions from org.apache.spark.sql.functions._, if option is set to false. Default will be true, which is current default behaviour.

Motivation: sometimes I import implicits that are in conflict with SQLImplicits, e.g. when importing Catalyst DSL, and would prefer to turn it off for these cases.