Only apply 'width' for Textareas; Input fields are effectively infinite width

If you leave the width restriction off non-textarea fields (e.g. input[type=text] fields), then you get a closer approximation of sizing, which results in more accurate position calculation based on the Mirror.

This does *not* fix the problem of input fields calculating position incorrectly, but it improves it significantly. If an input field has a very long input, eventually the position is still calculated incorrectly using this method.

See [#17](https://github.com/ichord/Caret.js/issues/17)