Add Blink method to textinput to return blink state

[aschey]

As discussed on Slack earlier, this PR adds a Blink() method that returns the text input's blink state. This is useful if you want to render something more complex than the existing View() method can handle. For context, I'm implementing an input with multiple types of placeholders, each with different styles. I thought about adding something to the text input itself to handle this case, but it would add a lot of complexity for a rather niche use case. With the blink property exposed, users can implement their own View() method.

[meowgorithm]

One small comment above, otherwise we're good to go. Thanks for this one!