Updates and refactors spectrum.zsh

Minor stuff.

Deletes a line that looks like remnants of an older try to implement the method (l. 32) and provides more text instead of the plain 'Test'. I think the colors are easier to distinct visually if you see a longer string.

If people are funny they can provide a custom string in ZSH_SPECTRUM_TEXT and have that displayed in spectrum functions.

Closes #2577