fix(input): character count hint for input

[davidcraddock]

fix (input): Remove the property CharacterCounter on md-input. Add name
of local variable on md-input and add md-hint to fix counter binding of
md-input for the show hint lable with charater count example.

The property characterCounter does not exist on md-input and therefore
the character count was not being displayed for the show hint lable with
charater count example. Removing this and adding the name
#characterCountHintExampleInput along with the md-hint tag using the
name and the character count property fixes the issue.

[jelbourn]

LGTM

[dahaupt]

@jelbourn Is this really merge ready? He should rebase first because of the deprecated md-input tag. Sorry if I am wrong about this.

[jelbourn]

GitHub says merge base is clean. This demo will go away before too long, but in the meantime the change is still correct

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