feat(stepper): add the ability to reset a stepper

[crisbeto]

Allows for the user to reset a stepper to its initial state by calling the reset method.

Relates to #7700.

[crisbeto]

Note: I'm open to having this reset only the dirty and touched states. I went with reset, because leaving behind the value didn't seem like a "true" reset.

[mmalerba]

This seems like the most common use case to me. If we get lot of requests for reset all but the value we could create a new method: {reset,clear}{Status,State}

[mmalerba]

This seems like the most common use case to me. If we get lot of requests for reset all but the value we could create a new method: {reset,clear}{Status,State}

[mmalerba]

@crisbeto I set the milestone to 5.2 since this seems less severe than some of the other things we want to get in for 5.1. I'd be fine moving it to 5.1 if we have time after merging the other fixes

[crisbeto]

I'm fine with leaving it for 5.2.

[andrewseguin]

See CI errors

Error at /home/travis/build/angular/material2/dist/packages/e2e-app/material-examples/typings/index.d.ts.StepperOverviewExample.html(1,1030): Property 'matHorizontalStepper' does not exist on type 'StepperOverviewExample'.

[crisbeto]

Sorted out the error in the example.

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