Randomly pick from caller-passed list.

[other-mickk]

Split out some of the #random() functionality into a #random_among(choices) function, to switch to a colorscheme from a caller-passed argument list.

In a perfect-world we'd have even better separation of concerns:

• random number generation (or better, have it built-in into Vim)
• graceful switching of colorschemes
• management of colorschemes: randomly pick from a list (from all schemes, from a shortlist, from a user-supplied list, etc.) according to some rules (excluding schemes from a blacklist, in a background-preserving manner, etc.)

This PR is not that ambitious however. Just having #random_among(choices) makes it easier e.g. for me to implement vim-colorscheme-manager features, albeit at the cost of intertwining the two plugins.