[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,
         })

bumblebee/theme.py

@@ -74,6 +74,14 @@ class Theme(object):
         """Return the background color for this widget"""
         return self._get(widget, "bg", None)
 
+    def align(self, widget):
+        """Return the widget alignment"""
+        return self._get(widget, "align", None)
+
+    def minwidth(self, widget):
+        """Return the minimum width string for this widget"""
+        return self._get(widget, "minwidth", "")
+
     def separator(self, widget):
         """Return the separator between widgets"""
         return self._get(widget, "separator", None)
@@ -131,6 +139,7 @@ class Theme(object):
                 state_themes.append(self._get(widget, state, {}))
 
         value = self._defaults.get(name, default)
+        value = widget.get("theme-{}".format(name), value)
         value = self._cycle.get(name, value)
         value = module_theme.get(name, value)