New mixin for offcanvas hamburger icon

Creates hamburger icon for offcanvas menu
By default it creates the same styling as of 5.1.1

```$tabbar-hamburger-icon-left``` and  ``` $tabbar-hamburger-icon-top```  are set to false by default, as a result the icon is centred both horizontally and vertically in the offcanvas icon. 
If a value is specified then respective centring in disabled

Also you can specify the thickness of hamburger icon lines as well as the gap or spacing between the lines using ```$tapbar-hamburger-icon-thicknes``` and ```$tapbar-hamburger-icon-gap```