ENH: StringMethods now supports ljust and rjust
Derived from #9111.
- Add `StringMethods.ljust` and `StringMethods.rjust` to be consistent with standard `str`.
- Add `fillchar` option to `StringMethods.pad` and `StringMethods.center` to be consistent with standard `str.center`
  - https://docs.python.org/2/library/string.html#string.center
