[Ruby] 빈 공간 텍스트 채우기

"hello".ljust(4)                  #=> "hello"

"hello".ljust(20)                #=> "hello               "

"hello".ljust(20, '1234')    #=> "hello123412341234123"

"hello".rjust(20, '1234')    #=> "123412341234123hello"