[modules] Separate modules into core & contrib

Also, improve errors when importing a module fails.

Also, add more tests.

modules/core/date.py

@@ -0,0 +1,19 @@
+# pylint: disable=C0111,R0903
+
+'''Displays the current date and time.
+
+Parameters:
+    * date.format: strftime()-compatible formatting string
+    * date.locale: locale to use rather than the system default
+'''
+
+from .datetime import Module
+
+class Module(Module):
+    def __init__(self, config):
+        super().__init__(config)
+
+    def default_format(self):
+        return '%x'
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4