Add --prompt2 and --no-prompt2

[apstndb]

The default behavior is unchanged.

$ ./spanner-cli --project ${SPANNER_PROJECT} --instance ${SPANNER_INSTANCE} --database ${SPANNER_DATABASE}             
Connected.
spanner> SELECT 1 +
      -> 2;

--prompt2 modifies multi-line prompt.

$ ./spanner-cli --project ${SPANNER_PROJECT} --instance ${SPANNER_INSTANCE} --database ${SPANNER_DATABASE} --prompt2='$ '
Connected.
spanner> SELECT 1 +
       $ 2;

--no-promp2 sets prompt2 to empty.

$ ./spanner-cli --project ${SPANNER_PROJECT} --instance ${SPANNER_INSTANCE} --database ${SPANNER_DATABASE} --no-prompt2     
Connected.
spanner> SELECT 1 +
         2;

fixes #183

[apstndb]

My latest thought is

• In the future, PostgreSQL style prompt variables such as %R may be supported, in which case --prompt2 will be required.
• --no-prompt2 is not needed if --prompt2 can differentiate between unset and empty
  • It seems that jessevdk/go-flags can do it using *string.

It seems to be better to unify to single option --prompt2.

[apstndb]

I have prototyped another implementation with parser status
#185