repl: improve .help message

- Added dots to printed commands.
- Use spaces instead of tabs so there's no misalignment on terminals
  with a tab size other than 4.
- Improved the help text for .editor and .help.
- Automatically indent command help based on the longest command.

PR-URL: https://github.com/nodejs/node/pull/8519


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>