Dark theme for jetzt speedreader

Added a dark theme for jetzt, taking into account comments from ds300. Trying
to stay truer to the control methods have already been written. Changed the
way the CSS is implemented, to instead add the sr-dark class to the sr-reader
element.
Merge pull request #33 from ianzapolsky/dark-theme

Dark theme for jetzt speedreader
update readme with dark theme usage
added default dark option to avoid warnings