Add linter to font_awesome_flutter

Added Flutter/Dart lints to project. Fixed all warnings except example which won't patch correctly if changed.

Thanks to @gslender

Enable/Disable icon styles automatically

Based on availability and excludes list

Old file path to tool/ folder -> new is util/

Added option to generate icon mapping

Fixes #174, #172, #102, #71, #10 and possibly more...

Refactor tools

5.15.4

[Minor nit] Update CHANGELOG.md

Fix spacing on CHANGELOG