Add new option --with-plugins to use plugins in cli().

e.x pgrest --with-plugins websocket passport
Merge branch 'master' into feature/plugin-mechanism-next
make plv8x functions are also valid in a plugin.
Add new option --with-plugins to use plugins in cli().

e.x pgrest --with-plugins websocket passport
make plv8x functions are also valid in a plugin.
Merge branch 'feature/plugin-mechanism-next' of https://github.com/pgrest/pgrest into feature/plugin-mechanism-next