Remove usage of mdi-* icons

[frankwong15]

mdi-* icons are phasing out as per Dogfalo/materialize#1824

To maintain backward compatibility, maybe we can add mdIcon parameter?

{{md-btn text="Button" icon="mdi-action-favorite"}}

to become

{{md-btn text="Button" mdIcon="favorite"}}

and

<i class="mdi-action-favorite"></i>

to become

<i class="material-icons>favorite</i>

[mike-north]

If we add in backward compatibility, I'd like for it to be deprecated immediately. In the future, hopefully everyone will be using the new material-design-icons

[frankwong15]

Do we want to remove the icon parameter completely? I think it is handy if someone want to use Font Awesome icon instead.

For example

{{md-btn text="Github" icon="fa fa-github"}}

[mike-north]

Using contextual components allows for some reasonable options w.r.t this, but that'll have to wait for v1.0, since the minimum ember version we need is 2.3.