Add avocados, herbs, garlic and shallot to the bowl of a food processor.Zest the lime into the processor then add its juice, too.Blend everything until smooth. Thin with water, additional citrus juice, or milk if desired.Store in an airtight container in the refrigerator. To store more than 1 day, squeeze more citrus over the top to keep it from browning and stir this in before use.