Added option to output JSON file describing generated font glyphs and their unicode positions

Useful for providing data to run custom templates after generating new font.

Addresses the underlying issues from #39, #48 and #50

This approach would improve separation of concerns: FontCustom generates font files and data describing the generated font. Responsibility for CSS style/framework, test files, documentation generation, etc can be moved away from FontCustom and the JSON file used to populate any sort of template required, in any language or build system the user requires.