Add CSS for Genesis skip links

[salcode]

See http://genesis-accessible.org/2015/07/genesis-2-2-accessibility-changes-and-features/

/* # Skip Links
---------------------------------------------------------------------------------------------------- */
.genesis-skip-link {
 margin: 0;
}
.genesis-skip-link li {
 height: 0;
 width: 0;
 list-style: none;
}
/* Display outline on focus */
:focus {
 color: #333;
 outline: #ccc solid 1px;
}