added a function to allow user to use a mixin to insert a single icon into compiled CSS instead of the full icon list, therefore allowing customization without editing icon list.
Merge branch 'icons' of github.com:wadehammes/Gumby into wadehammes-icons
Added option to specify multiple icons or 'all', moved icon function/mixins to functions directory, moved entypo import to gumby.scss and used icons mixin in _base.scss
Merge branch 'wadehammes-icons' into develop