[core/config] enable logging into a log file

Re-enable the -f | --logfile parameter, but still default to logging to
stderr.

Also, add a "debug" module that is automatically displayed if a bar is
run in debug mode (thanks to @bbernhard for the excellent suggestion)

fixes #614

modules/core/debug.py

@@ -0,0 +1,22 @@
+# pylint: disable=C0111,R0903
+
+"""Shows that debug is enabled"""
+
+import platform
+
+import core.module
+import core.widget
+import core.decorators
+
+class Module(core.module.Module):
+    @core.decorators.every(minutes=60)
+    def __init__(self, config, theme):
+        super().__init__(config, theme, core.widget.Widget(self.full_text))
+
+    def full_text(self, widgets):
+        return 'debug'
+
+    def state(self, widget):
+        return 'warning'
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4