Add --enable-dist-8x8, rename existing '--tune' as '--tune-metric'
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