WIP: md-input directive

Old problem: We didn't know what attributes to pass down to the input directive from the input container.

Solution: We have a directive with priority 100 and `terminal: true`. `terminal` means that no directive with lower priority will run on this element - basically, directive compilation will stop at `md-input`.

With `priority: 100`, `md-input` still runs after `ng-if`, `ng-repeat`, etc.

When `md-input` runs, it takes all attributes on `<md-input>` container and copies them down to a child `<input>` element - EXCEPT attributes that are directives with higher priority (like ng-if).

**PROBLEMS**

- ng-show is actually 0 priority, so it will be passed down to the input element. That means the input element will be hidden, but not the parent.
You're right, I should be using the `compile` function to get the un-interpolated values.