v0.0.19

• Increased specificity of CSS rules for the panelset.

• Added --panel-tab-background-color with additional -active and -hover
  custom properties to style the background color of tabs. Similarly, two
  CSS variables for border color were added: --panel-tab-border-color-active
  and --panel-tab-border-color-hover.

• Updated Tachyons to 4.12.0 (#27)

• Added clipboard extra with use_clipboard(). Adds a "Copy Code" button
  to code chunks (<pre><code>...</code></pre>) and works in both xaringan
  slides and R Markdown documents (thanks @mattwrkentin #20, #36).