Add repeat_directive::operator() to allow repeat with delimiter

[andreasbuhr]

Implement request in #231

I think this is what @psiha is asking for.

I implemented it in operator() to pass two parsers to repeat_directive. This is not so nice. But with operator[] we get only one argument. Do you have any other idea?

[tzlaine]

I'm not sure about this one. It looks completely different from other directives. The [] is a pretty important clue how to read a parser. This might be something to push until C++23 is more widespread. That allows multiple op[] params.