Added image optimization with ImageOptim

On MacOS, Inkcite now supports running its images through ImageOptim when building the production version of the email.  Added optimize-images configuration to control this behavior.

The Build CLI logic was updated to always compile to the build directory.  Optimized images are copied into that directory after the HTML has been assembled.  Changed the archive function to read the files from the build directory rather than having a separate flow requiring duplicate code.