Allow newline in prompt

[apstndb]

Other interactive SQL tools (and interactive shells) allow to contain newlines in prompt, and it is used by some users.

Examples

• https://superuser.com/questions/633766/how-do-i-display-a-newline-in-a-mysql-prompt
• https://www.postgresql.org/message-id/E1j0wwc-0003qZ-PW%40gemulon.postgresql.org

I believe that allowing newlines helps both showing detail informations in prompt and short indentation in spanner-cli.

Expected behavior

prompt = "[\\p:\\i:\\d]\\n   > "

shows

[very-long-project-id:very-long-instance-id:very-long-database-id]
   >

and continuation prompt is aligned to the last line of the prompt

[very-long-project-id:very-long-instance-id:very-long-database-id]
   > SELECT *
  -> FROM Singers;

Related discussion #183 (comment)