Bootstrap 4!

Added support for Bootstrap 4. See the README for details on how to migrate.

Basically new CSS:

/* Bootstrap 4 Differences */
#scrollspy>ul.nav {
  /* for affix if wanted */
  position: fixed;
  /* vertical orientation */
  flex-direction: column;
}
/* bootstrap 4 puts the scrollspy .active on
     the <a> instead of the <li> item */
.nav>li>a.active {
    font-weight: bold;
    border-left: 2px solid gray;
}
/* need to see the child <ul> */
.nav-link.active+ul.nav.child {
    display: block;
}