Enable users to customize initial commands.

move initialCommands to the external file, init/init.scala.
Files under init/ are loaded when starting dbconsole.

```
├── build.sbt
├── config.properties
├── dbconsole
├── init
│   ├── util.scala // created by user
│   ├── helper.scala  // created by user
│   └── init.scala

```