[modules] Add module-specific configuration

Big oversight in my previous commits: Widgets need to be able to have
specific configurations (i.e. the path for different instances of the
"disk" module has to be different).

To account for that, it is now possible to assign an "alias" to a module
instance using ":" (for example: -m "disk:home"). This alias is then
used for the configuration parameter resolution automatically, for
example:

-m disk:home -p home.path=/home

As a consequence, parameter names in the module code are now relative to
the module, which means: shorter!

bumblebee/module.py

@@ -2,6 +2,7 @@ import os
 import pkgutil
 import importlib
 
+import bumblebee.config
 import bumblebee.modules
 
 def modules():
@@ -32,9 +33,11 @@ class ModuleDescription(object):
         return "n/a"
 
 class Module(object):
-    def __init__(self, output, config):
-        self._config = config
+    def __init__(self, output, config, alias=None):
         self._output = output
+        self._alias = alias
+        name = "{}.".format(alias if alias else self.__module__.split(".")[-1])
+        self._config = bumblebee.config.ModuleConfig(config, name)
 
     def critical(self, widget):
         return False
@@ -46,6 +49,6 @@ class Module(object):
         return "default"
 
     def instance(self, widget):
-        return None
+        return self._alias if self._alias else self.__module__.split(".")[-1]
 
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