pp prints a multiple lines string prettier.

• lib/pp.rb (String#pretty_print): Defined to print a string as
  multiple lines.
  [ruby-core:76800] [Feature#12664] proposed by Petr Chalupa.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56391 b2dd03c8-39d4-4d8f-98ff-823fe69b080e