Add none theme

[segevfiner]

Fixes #1120

[segevfiner]

Vanilla CSS for those who need it:

.tippy-box[data-theme~='none'] {
  background-color: transparent;
  color: inherit;
}

.tippy-box[data-theme~='none'][data-placement^='top'] > .tippy-arrow::before {
  border-top-color: transparent;
}
.tippy-box[data-theme~='none'][data-placement^='bottom'] > .tippy-arrow::before {
  border-bottom-color: transparent;
}
.tippy-box[data-theme~='none'][data-placement^='left'] > .tippy-arrow::before {
  border-left-color: transparent;
}
.tippy-box[data-theme~='none'][data-placement^='right'] > .tippy-arrow::before {
  border-right-color: transparent;
}

.tippy-box[data-theme~='none'] > .tippy-backdrop {
  background-color: transparent;
}

.tippy-box[data-theme~='none'] > .tippy-svg-arrow {
  fill: transparent;
}