Added 'cdef-dist' and 'daala-dist' options to --tune-metric'.
(Actual bodies of the options will be added in later separate commits)

Also, if --tune=cdef-dist or --tune=daala-dist,
enable-dist-8x8 is automatically turned on.

This commit is a part of prep-work to remove DIST_8X8, CDEF_DIST,
and DAALA_DIST experimental flags.

Change-Id: I0790b9537dfaf166cb59741997ae27da10697a33