Add --output-file option to mako-render

Added --output-file argument to the Mako command line runner, which allows
a specific output file to be selected.  Pull request courtesy Björn
Dahlgren.

Fixes: #283
Change-Id: Iae7e2d42d5ae4bc6f09663c115bda9e3797ce68c
Pull-request: https://bitbucket.org/zzzeek/mako/pull-requests/27