Allow hamburger to be resized by changing as few values as possible

[chrisdwheatley]

Currently it's more difficult than it should be to resize the hamburger. Need to figure out a way to make it easier to resize for users.

[paulomcnally]

This work for me:

.material-design-hamburger__layer
    width: 20px;
    height: 2px;

.material-design-hamburger__layer:before, .material-design-hamburger__layer:after
    height: 2px;