New macro "include_example":

- quotes all uncommented lines in a code block
- comment lines that start with "#@" (doxygen) are quoted as documentation
  blocks
- lines can be skipped via "#[%-%]"/"#[%+%]" markers

Replaces "include" macro in practice.
Strip parameters from trailing whitespace properly.
New macro that includes a snippet between a begin and end regex as code.

If begin and end are comments, skip those.