Material Stepper icon change

[AnandMani1]

Bug, feature request, or proposal:

feature request

What is the expected behavior?

need to the stepper index number to alphabetic

What is the current behavior?

index is coming only in numbers.

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

What is the use-case or motivation for changing an existing behavior?

for sub-steps i need to change the numbers into alphabetics.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

angular 5, material-5.2.4

Is there anything else we should know?

[crisbeto]

This will be possible once #10516 is released.

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