Adds the ability to use starts_with and ends_with in string filters

filter :title_starts_with, :as => :string
filter :title_ends_with, :as => :string

https://github.com/gregbell/active_admin/issues/1958
Merge pull request #1962 from howaboutwe/1958_allow_filtering_with_starts_with
Updated CHANGELOG
Merge branch 'master' of https://github.com/gregbell/active_admin