WIP: try to parse content of org blocks natively.

[Fuco1]

Fixes #150

The implementation is a dirty special-case for org. A better solution is required but for now this branch can be used by people to try it out.

We should do something about #903 first and then on top of that build "buffer-local" versions of the parser.

• sp-show--pair-function does not know how to pick up the delimiters which are not defined in org (such as a def-end in a ruby block.
  • add a wrapper macro "parse-in-buffer-with-major-mode` which will do the setup and run the parser there.
• wrap the internal api in a nice public api with per-buffer overridable versions. The internal parsing functions should not be touched but only called in a proper context.
• add functions/api to modify the sp-get compatible objects. For now, we need at least something that can add an offset to :beg and :end.