Add String#rjust to mruby-string-ext

mrbgems/mruby-string-ext/mrblib/string.rb

@@ -287,6 +287,29 @@ class String
     return newstr.slice(0,idx)
   end
 
+  ##
+  #  call-seq:
+  #     str.rjust(integer, padstr=' ')   -> new_str
+  #
+  #  If <i>integer</i> is greater than the length of <i>str</i>, returns a new
+  #  <code>String</code> of length <i>integer</i> with <i>str</i> right justified
+  #  and padded with <i>padstr</i>; otherwise, returns <i>str</i>.
+  #
+  #     "hello".rjust(4)            #=> "hello"
+  #     "hello".rjust(20)           #=> "               hello"
+  #     "hello".rjust(20, '1234')   #=> "123412341234123hello"
+  def rjust(idx, padstr = ' ')
+    if idx <= self.size
+      return self
+    end
+      padsize = idx - self.size
+      newstr = padstr.dup
+      while newstr.size <= padsize
+        newstr << padstr
+      end
+    return newstr.slice(0,padsize) + self
+  end
+
   #     str.upto(other_str, exclusive=false) {|s| block }   -> str
   #     str.upto(other_str, exclusive=false)                -> an_enumerator
   #