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

[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:

bootstrap/scss/_forms.scss

Lines 110 to 127 in d650197

	.form-control-plaintext {
	display: block;
	width: 100%;
	padding-top: $input-padding-y;
	padding-bottom: $input-padding-y;
	margin-bottom: 0; // match inputs if this class comes on inputs with default margins
	line-height: $input-line-height;
	color: $input-plaintext-color;
	background-color: transparent;
	border: solid transparent;
	border-width: $input-border-width 0;
	&.form-control-sm,
	&.form-control-lg {
	padding-right: 0;
	padding-left: 0;
	}
	}