ENH: global value is configurable from the command line and the configuration file.

Usage:
    baw_exp.py --ignore_datasinks ...
       Sets GLOBAL_DATA_SINK_REWRITE == False

    In configuration file:
       [PIPELINE]
       IGNORE_DATASINKS = True

       Sets GLOBAL_DATA_SINK_REWRITE == False
Merge pull request #175 from dmwelch/enh/GLOBAL_DATA_SINK_REWRITE

ENH: GLOBAL_DATA_SINK_REWRITE