[rss] Small fixes

- White foreground text now works
- Feed list title filtering is now case-insensitive
[rss/clock] Add time format and time zone config options

The clock module can now set global config options for time format and
time zone. The time command will look for user or channel preferences
first, and then for these config options.

The RSS module now reads time format from the same place: it will look
for a channel preference, then for the config option.