Stop setting icons through method_missing

[Angelmmiguel]

As today, the way to set the icon in a specific material icons instance is to call a method:

mi.calendar

This approach is based on the special method_missing method from Ruby. At the beginning, I decided to use this approach as it simplifies the way you set the icon. However, I quickly noticed some drawbacks:

• You miss real exceptions when calling methods that doesn't exist in that class. This may make more difficult to debug certain issues
• There are icons that starts with a number, which it's invalid for ruby methods

Now, checking the new PR to upgrade the gem to 4.0.0, I noticed many new icons starting with a number. For this reason, I plan to move from this approach to a specific method in v4.0.0.

Note this is a breaking change and already existing users would need to change their code:

# v2.2.0 and before
mi.calendar

# v4.0.0
mi.shape(:calendar)
mi.shape('calendar')