Fix incorrect behavior in argv.StoreInModule (#389)

Also document it now that it works :)

Previously, this would always try to set attributes in the openhtf module itself, not openhtf.util.logs or whichever module was correct.

openhtf/util/argv.py

@@ -1,4 +1,20 @@
-"""Utilities for handling command line arguments."""
+"""Utilities for handling command line arguments.
+
+StoreInModule:
+  Enables emulating a gflags-esque API (flag affects global value), but one
+  doesn't necessarily need to use flags to set values.
+  
+  Example usage:
+    DEFAULT_VALUE = 0
+    ARG_PARSER = argv.ModuleParser()
+    ARG_PARSER.add_argument(
+        '--override-value', action=argv.StoreInModule,
+        default=DEFAULT_VALUE, target='%s.DEFAULT_VALUE' % __name__)
+
+  Then in an entry point (main() function), use that parser as a parent:
+    parser = argparse.ArgumentParser(parents=[other_module.ARG_PARSER])
+    parser.parse_args()
+"""
 
 import argparse
 
@@ -19,5 +35,7 @@ def __init__(self, *args, **kwargs):
     def __call__(self, parser, namespace, values, option_string=None):
         if hasattr(self, '_proxy'):
             values = self._proxy(parser, namespace, values)
-        setattr(__import__(self._tgt_mod), self._tgt_attr, values)
+        base, mod = self._tgt_mod.rsplit('.', 1)
+        module = getattr(__import__(base, fromlist=[mod]), mod)
+        setattr(module, self._tgt_attr, values)