Add the 'class_icon' method because we could not use an icon named 'class'

[ykpythemind]

I could not use this icon named 'class', because name was duplicated with Object#class method.

https://material.io/tools/icons/?search=class&icon=class&style=baseline

So, calling mi.class_icon now sets the class icon.

[coveralls]

Coverage increased (+0.1%) to 98.618% when pulling 49db5a3 on ykpythemind:class_icon into c92a255 on Angelmmiguel:master.

[Angelmmiguel]

You're right, icon names are a problem with the current approach of method_missing. From time to time the gem faces this kind of issues.

Thank you very much for your PR! 👍