library: require .scad, suggest correct method when typo, add support for importing .scad files, prism primative aliases.

Now you can import .scad files using ruby's require(). Oozby will scan the .scad file and recognise the modules available, enabling them in the local interpreter, while still throwing errors for modules not in that file tree.

Using an unknown method (like sine() for instance) results in clearer error message explaining you may have wanted sin(), where before there was no error - openscad wouldn't error either usually, just nothing would render!

In addition to hexagon and friends, now we have hexagonal_prism and friends, which take a 'height' option. ^_^

Starting to document lib/oozby/method_preprocessor.rb for hackers wanting to experiment adding features.
lib: method_preprocessor additions to support friendly errors
cli: improvements for friendlier error reporting
examples: demo requiring a scad file and using it's modules