Add btn-outline-*-inverse for dark backgrounds

Created by: IamManchanda

btn-outline-* is a great option but adding inverse classes ( btn-outline-*-inverse ) will help buttons on dark backgrounds to be presented better

for eg.

btn-outline-primary-inverse {
    color: #fff;
    background-image: none;
    background-color: transparent;
    border-color: #fff;
}
.btn-outline-primary-inverse:hover {
    color: #fff;
    background-color: #0275d8;
    border-color: #0275d8;
}
.btn-outline-primary-inverse:focus, .btn-outline-primary-inverse.focus {
    color: #fff;
    background-color: #0275d8;
    border-color: #0275d8;
}

Adding some screenshots for the demo purposes