[Bootstrap v4] Generate Tooltips from the $theme-colors map

[andreivictor]

As seen on:
https://getbootstrap.com/docs/4.4/getting-started/theming/#components

Apply the same logic that is used by Bootstrap team to generate modifiers to the components (alerts, badges, buttons).

We'll generate tooltips from the $theme-colors map:

@each $color, $value in $theme-colors {
  .tooltip-#{$color} {
    @include tooltip-custom($value, color-yiq($value));
  }
}

[andreivictor]

Implemented in #11.