64 automatic dark mode for pictures

[Benno2000]

I implemented the \picDark command from the SPL slides. It can be used to include images using \pic but automatically inverting them in dark mode (white gets inverted to a dark gray that matches the slide background).

This therefore closes issue #64.

[tthuem]

Demo file misses the case of "-dark" files.

[Benno2000]

@tthuem I updated the example in the demo slides accordingly. It now mentions the option to use "-dark" files in the explanation.

[tthuem]

Fine with me. You might want to squash all commits into a single commit.