Implement support of the new context menu API

Starting with atom/atom#3658, a new API allow to make context menu
display dynamic.
:lipstick: Use => for shouldDisplay
:fire: Remove obsolete assignment
Merge branch 'feature/context_menu_api' of https://github.com/abe33/color-picker into abe33-feature/context_menu_api

Conflicts:
	lib/ColorPicker.coffee
Merge branch 'abe33-feature/context_menu_api'