This validator lets the user write either a real list, like [a, b, c], or just
a whitespace-separated string, like a b c. This is a little nicer for some
settings like "plugins" where the brackets and commas just look like line
noise.
@tezoulbr: I'm changing these to debug messages partially so they don't print
out when running the tests (with nose, for example) but also because it could
get a little annoying for someone who *intends* to use the defaults for one of
these plugins. Let me know if you disagree.
