Sync plain text font-size with $input-font-size

Created by: ysds

If change the $input-font-size, but the font size of .form-control-plaintext doesn't change.

demo: https://codepen.io/anon/pen/YgrJQW (In this demo, the custom font sizes are intentionally specified huge values.)

Note: The .form-control-plaintext with size options are not documented but they are supported in the source:

https://github.com/twbs/bootstrap/blob/d6501970adea3a869bf1b204eba2a7710236dc7a/scss/_forms.scss#L110-L127