Allow to enter multi-line BQL queries in the shell

[dnicolodi]

This most likely requires moving away from a simple cmd.Cmd interpreter loop and it requires the parser to be able to detect incomplete commads. It may be very difficult to achieve this without requiring BQL statements to be terminated by a semicolon, because the SQL grammar is otherwise ambiguous: there is not way to know if SELECT 1 is a standalone statement ot is the begginning of a more comple statement SELECT 1 FROM x WHERE ...