value-pairs: Support string list in key() and exclude()

This adds support to value-pairs for using multiple elements in the
key() and exclude() options. This makes it easier to wrap them in an SCL
block. Same feature was added to the command-line parser too.

These partially address #219.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #220 from algernon/f/value-pairs/string-list-for-globs

value-pairs: Support string list in key() and exclude()