Adding string.ljust(), string.rjust() and string.center()

* `string.ljust(inp, width[, char])`  - returns left-justified string
  filled with charachter 'char' (' ' by default).
* `string.rjust(inp, width[, char])`  - returns right-justified string
  filled with charachter 'char' (' ' by default)
* `string.center(inp, width[, char])` - returns centered string
  filled with charachter 'char' (' ' by default)

closes gh-2214