Added ability to change placeholder opacity

This is my first 'Fork' so not sure if I have done it right?
I have added the ability to change the opacity of the placeholder text as well as extending this so you can actually change the color of the placholder on :focus.


For Example;

input[type=text] {
   @include placeholder($text-color);
   
   &:focus {
       @include placeholder($white);
       background-color: $base-color;
   }
}