cli: Only escape back-slashes in the player command on Windows.

This makes it slighly less confusing to use player
paths containing spaces on *nix.

E.g. you used to have to do:

    --player "'/path/with spaces/player' --args"

Now you can do this aswell:

    --player "/path/with\ spaces/player --args"
docs: Add QuickTime to players page.

Resolves #450.