Be more flexible in finding solution delimeters

[jhamrick]

See the discussion in #542

This would essentially revert #523, which is unfortunate in that it will result in yet another API change, but I think it will ultimately make things simpler as @dsblank suggests.

[jhamrick]

Probably whatever PR implements this should also tackle #422 too.

[dsblank]

You could probably still allow the old delimiters to work too, and slowly deprecate them.

[jhamrick]

Yes, though the way the current delimeters work hasn't been in an official release yet, so I'm more ok with breaking those.