[core] Added min-width and alignment themeing

Added theme-options ("minwidth" and "align") for setting the minimum
width and the alignment of a widget.

Also, allow widget to provide defaults for the theme options by setting
an attribute in their store called "theme-<name of the theme option>".

For example, a widget can now define a default alignment by using:
widget.set("theme-align", "default-value").

bumblebee/output.py

@@ -89,6 +89,8 @@ class I3BarOutput(object):
             "background": self._theme.bg(widget),
             "separator_block_width": self._theme.separator_block_width(widget),
             "separator": True if separator is None else False,
+            "min_width": self._theme.minwidth(widget),
+            "align": self._theme.align(widget),
             "instance": widget.id,
             "name": module.id,
         })