Add customized separators:
* The default separators are automatically disabled if custom separators
are used (to "just" disable the default, use empty custom separators)
* Use previous background color as their background color and the
current background color as foreground color
* Allow the separator-block-width to be configured
see #23
Allow a theme to define a "cycle" of attributes that are cycled through
on a widget-per-widget basis (e.g. for alternating the widget
background). These cycles take precedence over the default values, but
can be overridden by module-specific theme instructions.
see #23
Allow sub-themes ("iconsets") to be merged into the "main" theme. That
way, effectively, it's possible to define colors and icons in separate
JSON files.
see #23
