appleseed.cli output filename Frame param

Related to issue #461.
This PR add an output filename parameter to the Frame entity in the project and an optional one to enable saving aovs.

"cli_output_filename" -> string
"cli_output_aovs" -> bool, false if not present.

The idea is to make it possible to specify an output filename in the project file.

This parameter only applies to appleseed.cli. appleseed.studio has a well defined way of saving images and should only save them when requested. A similar thing can be said of clients of the appleseed library.

Now, inside appleseed.cli, this param should have the lowest priority:

- If the user specifies a filename on the command line, cli should save images where the user requested and ignore the output param.

- If a display driver is specified in the project or another output choosen (for example mplay), cli should also do what the user wants and ignore the param.

The comment in issue #461 about After Effects not being able to read the sequences is unrelated to this  and should be handled in Frame::write_aov_images() instead.