Extend auto-complete for db/table names and subcommands

[ravi]

Currently at the interactive prompt, some autocomplete works (e.g: r.db<tab> shows the various db commands such as dbCreate, dbList, etc). It would be nice if this can be extended to support the full range of auto-completion available in the Web UI Data Explorer:

1. completion for DB and table names
2. completion beyond the first level: r.db('mydb').ta

[ravi]

I am hoping that since the WebUI (DataExplorer) is JS, some of that code can be borrowed directly for auto-completion needs.