Added support for fixed unicode character per glyph.

lib/fontcustom/scripts/generate.py has been updated to read glyph settings from glyphs.json - if it is available. The order of the glyphs and each glyphs unicode character are controlled by glyphs.json. Generate.py writes the unicode characters to .fontcustom-data which is then read by lib/fontcustom/generator/template.rb. The templates are using unicode characters read from .fontcustom-data.

Example icon fonts have been added – inluding an example of glyphs.json:
    1. example/testicon - Default templates and settings
    2. example/testicon-custom - Custom templates, fixed unicode characters read from glyphs.json