Regexp: %r notation bug fix. close #85.

be able to parse any delimiters.
variable expansion isn't supported yet.

 * sample code
```
def matchpath(str)
  puts "#{str} is ..."

  if %r!img/! =~ str then
    puts "match"
  else
    puts "not match"
  end
end

matchpath("img/a.png") # => match
matchpath("/usr/local/bin") # => not match
matchpath("book/img/local") # => match
```
Merge pull request #905 from iij/pr-percent-notation

Regexp: %r notation bug fix.