Bug 29033: Add C4::Context->multivalue_preference

Bug 29033: Add C4::Context->multivalue_preference

I've seen several places in which a syspref is retrieved and then
splitted using split and the fact they are pipe-separated strings.

It seems it would be simple (and handy) to add a method to do that.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/Context.t
=> SUCCESS: Tests pass, a pipe-separated syspref is correctly retrieved
as an arrayref.
3. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>