add --output-file option to mako-render

[zzzeek]

CC @bjodah per pull request submitted to bitbucket:

This option is useful in e.g. Makefiles where currently using a redirect to target creates a file even on rendering failure, e.g.:

target.txt: target.txt.mako
    mako-render $< >$@

this would allow a user to use a Makfile such as:

target.xt: target.txt.mako
    mako-render $< --output-file $@

which would only create the target upon successful rendering.

gerrit at https://gerrit.sqlalchemy.org/#/c/sqlalchemy/mako/+/958/, however we have no test fixture for the mako-render thing. that would be helpful to have.