Added 'List Functions' category, added `extract` and `length` functions
Moved `convert` function to 'Misc Functions', removed `color` and `unit` from 'Math Functions' (as they are already in 'Misc Functions')
Added `min` and `max` functions
functions.json: added `convert`, removed `color` duplicate; several minor formatting fixes in functions/*.md files
Merge pull request #29 from seven-phases-max/master

Added `min`, `max`, `extract`, `length` functions. A few function categories fixes.