[cli] add --core to pass arguments to the core

This adds a new commandline parameter to dt-cli, "--core". Everything
following this option will be ignored by dt-cli and just passed to the
core. Use at your own risk.

Sample usage:
darktable-cli IMG_1234.CR2 my_lovely_bw_style.xmp output.jpg --width
1200 --core -d perf -t 23