UMIP 87 -Add DIGG_Positive_Rebases as a supported price identifier

The DVM should support price requests for total DIGG_Positive_Rebases for 30 days after the launch of the options. A DIGG Positive Rebase can be determined if the total supply of DIGG ($35,243.00 | Digg (DIGG) Token Tracker | Etherscan) increased from one day to the next, or stayed at equilibrium.