More error-handing, and `font-display` support

• BREAKING: Change to get-token() return value when the key path cannot be
  resolved, and $handle-missing-keys is unset or silent: Maps along the
  missing key path are no longer interpolated into the returned string. This
  is part of the fix for #49. Use $handle-missing-keys for more precise
  missing subkey behavior.
  (Thanks to John Crim @johncrim)
• NEW/BREAKING: Font maps accept display key,
  which can be used to set font-display property for importing web-fonts.
  (thanks to André Schließer @schliesser)
• BUGFIX (#49): Using get-token() to resolve a missing key no longer errors
  when using dart-sass and $handle-missing-keys is unset or silent.
  Instead, the unresolved path is returned.
  (Thanks to John Crim @johncrim)
• BUGFIX (#61): Using get-token() to obtain a 2nd level or deeper submap no
  longer causes a fatal error when using dart-sass and $handle-missing-keys
  is null, warn, or error.
  (Thanks to John Crim @johncrim)
• INTERNAL: Run unit tests with dart-sass (in addition to node-sass, which was
  used prior).
  (Thanks to John Crim @johncrim)
• INTERNAL: Upgrade dev dependencies.
• DOCS (#53): Use html rather than :root for examples.
• DOCS: Wrong parameter name in contrast-ratio() function docs.