Bug 23580: Add C4::Context->yaml_preference

Bug 23580: Add C4::Context->yaml_preference

This patch adds a new methods to be used on YAML-based sysprefs. The
idea is that we would use it instead of handling the YAML decoding
in-place everywhere we use YAML syprefs.

Ideally, we could even move pipe separated values sysprefs into YAML,
etc.

To test:
- Apply this patches
- Run:
  $ kshell
 k$ prove t/Context.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>