Add `plainText` and `size` props for Input

[Igor Santos]

Read-only inputs which should not show as actual inputs have a different class, and currently it's not possible to implement them with the Input component - since using class="form-control form-control-plaintext" causes funny issues if you try to focus it haha

Also, I noticed the Input component is missing the size property, similar to what's already available in Button - it's not a mandatory prop as plainText would be, but nonetheless would help the Input feel more "at home" and paired with other components which have that prop.