Add prompt when in interactive mode. Fix Show Environment [num]

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add support for READLINE. Remark: it is not enabled by default. Rename tcmalloc option to TCMALLOC (using consistent name convention for cmake parameters).

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Change policy for adding input to readline history.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>