Don't create pyc unless told too

Generating pyc files is disabled unless the `write-pyc` flag is given.
This addresses #68.

invoke/cli.py

@@ -145,6 +145,12 @@ def parse(argv, collection=None, version=None):
             default=False,
             help="Enable debug output.",
         ),
+        Argument(
+            names=('write-pyc',),
+            kind=bool,
+            default=False,
+            help="Enable creating .pyc files.",
+        ),
     ))
     # 'core' will result an .unparsed attribute with what was left over.
     debug("Parsing initial context (core args)")
@@ -153,6 +159,10 @@ def parse(argv, collection=None, version=None):
     debug("After core-args pass, leftover argv: %r" % (core.unparsed,))
     args = core[0].args
 
+    # Disable creating .pyc files, unless the write-pyc flag was given.
+    if not args['write-pyc'].value:
+        sys.dont_write_bytecode = True
+
     # Enable debugging from here on out, if debug flag was given.
     if args.debug.value:
         enable_logging()