Add .help command.

[tzzed]

This PR fixes #158.
The .help command list all the available dot command.
A message Enter ".help" for usage hints is displayed at genji launch.

On-disk database using BoltDB engine at path my.db.
**.**
genji> .help
.exit 			        Exit genji program
.help 			        List all available dot commands
.indexes [table_name] 	Display all indexes of the database or that the given table_name
.tables 		                 List names of tables

[asdine]

Thanks! Just a few cosmetic changes

[asdine]

👍