feat(input): add ability to hide text divider

[soyzamudio]

@hansl @kara @jelbourn

Adds the ability to show or hide the underline beneath the input (just like with the floating placeholder)

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[soyzamudio]

I signed it!

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[peterblazejewicz]

That's called text divider, not underline.
Should it be allowed to hide per docs (just wondering)?
https://www.google.com/design/spec/components/text-fields.html#text-fields-input

[jelbourn]

@soyzamudio I chatted with @hansl about this and we agreed that this is something that would be better done with CSS than a new directive API. We want people to be able to do something like

.md-input-underline {
  display: none;
}

To hide this part of the component. Right now the view encapsulation makes this difficult, so we're going to change the component to ViewEncapsulation.None and document the approach once we have our more complete documentation.

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