[themes] Make it possible to merge themes to get "icon themes"

Add code that allows themes to be merged (i.e. if certain elements are
not present in a theme, another theme can be "overlaid" to add missing
elements).

Effectively, this is used to create the logical concept of an "icon
theme", which is loaded after the main theme. So, the main theme can
define colors, and the icon theme fills in any missing elements
(practically, all the icons in the form of prefixes and suffixes).

Icon sets are defined in a theme using the "icons" directive, which
should be an array.

see #17

bumblebee/theme.py

@@ -1,4 +1,5 @@
 import os
+import copy
 import json
 import yaml
 import glob
@@ -14,16 +15,44 @@ class Theme:
     def __init__(self, config):
         self._config = config
 
-        if os.path.isfile("{}/{}.yaml".format(getpath(), config.theme())):
-            with open("{}/{}.yaml".format(getpath(), config.theme())) as f:
-                self._data = yaml.load(f)
-        else:
-            with open("{}/{}.json".format(getpath(), config.theme())) as f:
-                self._data = json.load(f)
+        self._data = self.get_theme(config.theme())
+
+        for iconset in self._data.get("icons", []):
+            self.merge(self._data, self.get_theme(iconset))
+
         self._defaults = self._data.get("defaults", {})
         self._cycles = self._defaults.get("cycle", [])
         self.begin()
 
+    def get_theme(self, name):
+        for path in [ getpath(), "{}/icons/".format(getpath()) ]:
+            if os.path.isfile("{}/{}.yaml".format(path, name)):
+                with open("{}/{}.yaml".format(path, name)) as f:
+                    return yaml.load(f)
+            if os.path.isfile("{}/{}.json".format(path, name)):
+                with open("{}/{}.json".format(path, name)) as f:
+                    return json.load(f)
+        return None
+
+    # algorithm copied from
+    # http://blog.impressiver.com/post/31434674390/deep-merge-multiple-python-dicts
+    # nicely done :)
+    def merge(self, target, *args):
+        if len(args) > 1:
+            for item in args:
+                self.merge(item)
+            return target
+
+        item = args[0]
+        if not isinstance(item, dict):
+            return item
+        for key, value in item.iteritems():
+            if key in target and isinstance(target[key], dict):
+                self.merge(target[key], value)
+            else:
+                target[key] = copy.deepcopy(value)
+        return target
+
     def begin(self):
         self._config.set("theme.cycleidx", 0)
         self._cycle = self._cycles[0] if len(self._cycles) > 0 else {}