Improve optional settings

[hadimansouri]

To set an step as optional we have to add md-optional="any text" to the step directive. It make difficult making steps dynamically. I suggest to define optional steps as follow:

<md-step md-optional="{expression}" md-optional-text="Optional">

By this pattern we could dynamically define steps as optional. If expression is true step is optional and text could be show for it.