No support for global-variable-exists function

[matharden]

The global-variable-exists doesn't seem to exist and fails to compile the Sass when the tested global variable does not exist.

$blockquote-color: #ccc;  // Breaks when this is missing.

@mixin myblockquote {
  blockquote {
    @if global-variable-exists(blockquote-color) {
      background: $blockquote-color;
    }
  }
}

@include myblockquote;

[eevee]

Added this and the other *-exists functions, and pushed 1.3.5 to PyPI.