Setup slime contribs if set before loading.

[mtstickney]

In order to allow a customizable set of slime contribs to be setup when loading the helper, check whether slime-contribs has been set, and if so pass that list to slime-setup; otherwise fall back on the default '(slime-fancy) list.

slime-setup will use slime-contribs if it is called with no arguments, and the default value for slime-contribs is '(slime-fancy), so this could be done by calling slime-setup with no arguments. However, I'm not certain if that is the case with all relevant versions of slime, and we are already relying on the explicit-argument form, so that's the conservative choice here.

This should fix #10, if it's acceptable.