Display transparency / fix minor syntax error

For RGBA / HSLA capture groups, enable alpha channel and draw on top of a checkerboard pattern. Includes user option to disable, drawing all colors opaque, and option for alternate BG pattern. Current choices are the default 'mid' (~GIMP), and 'light' (~Photoshop). Closes #55.

Also adjusts `syntax()` function slightly to fail gracefully. Fixes #63.