Added section autonumbering

Added special tags for automagically including counters
* `~~~SECTION:MAJOR~~~` translates to section counter
* `~~~SECTION:MINOR~~~` translates to auto resetting in-section counter

This means that we can now replace our Lab/Exercise/Solutions headers
with macros, such as:
* `# Lab ~~~SECTION:MAJOR~~~.~~~SECTION:MINOR~~~: Lab Title`