New Ruby 2.1, 2.2, 2.3 Syntax

[korny]

Add support for new Ruby language features:

• [x] Ruby 2.1 number literal suffixes
• [x] Ruby 2.2 quoted hash keys (partial)
  • [x] simple strings
  • [ ] don't apply for old when a == "b": syntax (ignore, see https://github.com/rubychan/coderay-scanner-tests/commit/6ddf2400e344c32900263ae7a3b65003e38654b4)
  • [x] escape sequences
  • [x] simple interpolation
  • [ ] interpolation with nested {…} (too complicated)
• [x] Ruby 2.3 safe navigation operator
• [x] Ruby 2.3 squiggly heredoc

Use https://github.com/rubychan/coderay-scanner-tests/pull/8 for testing.