Button cleanup

[mdo]

• Buttons no longer have an attribute selector. Instead, we generate a grouped selector based on the variants you choose to include in $button-variants. This generates the following out of the box:

   .btn,
   .btn-solid,
   .btn-subtle,
   .btn-outline,
   .btn-text {
     /* … */
   }

• Input-based toggle buttons now put their inputs inside the <label>, with .btn-check going on the label. This reduces selector complexity—no more sibling selectors, ugh yay—and simplifies markup.

• Active, checked, and disabled states have been fixed.

• There's a new .btn-styled variant that replaces our previous global options method for styling buttons with gradients, shadows, etc. Now, for .btn-solid, add .btn-styled and you get beautiful buttons out of the box.

  

• New docs including a button playground for seeing how variant, size, and more affects buttons.

  

• New .btn-icon.

• Button sizes are now a Sass map—scales better for adding/removing. Now includes an xs button size again (shoutout old Bootstrap). XL is in there, but it's commented out in the Sass map—just felt like to much to provide out of the box.