Add option to skip the song in working memory

[rbong]

Added the option --skip-working to skip the song in working memory when the project is unspecified.

Let me know if you'd like the argument to be called something else. --skip-wm seemed too vague and --skip-working-memory seemed too long. I didn't add a short form because it seems hard to capture the meaning in a single character (-s also seems too vague).

The program fails when both --working-memory is given and --skip-working because it feels like that behaviour should explicitly be undefined. -i -1 does not fail because --name and --index both cause the option to be ignored.

Related to #65

[stijnfrishert]

First of all: thanks for wanting to work on liblsdj! Your help is very much appreciated :)

Your code looks perfectly fine to merge, though there's one small problem: This codebase uses the master branch for always up-to-date releases, and develop further on develop.

Would it be possible for you to merge develop into your branch, fix any merge conflicts, and request to merge this back in develop? I've started using a different third-party library for command-line argument parsing (to ditch Boost), so you may encounter some small differences there.

[rbong]

@stijnfrishert No problem, I've rebased the develop branch and fixed conflicts to keep the commit history clean and updated the target of this merge request. That should be better

[stijnfrishert]

Great! :D