Add new plugin for homebrew installed version of postgres

plugins/postgres/postgres.plugin.zsh

+# Aliases to stop, start and restart Postgres
+# Paths noted below are for Postgress installed via Homebrew on OSX
+
+alias startpost='pg_ctl -D /usr/local/var/postgres -l /usr/local/var/postgres/server.log start'
+alias stoppost='pg_ctl -D /usr/local/var/postgres stop -s -m fast'
+alias restartpost='stoppost && sleep 1 && startpost'
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