[core/theme] add xresources support

add support for reading foreground and background, and colors, from
xresources.py

many thanks to @Aliuakbar for the addition!

fixes #731

bumblebee_status/util/xresources.py

@@ -0,0 +1,10 @@
+import subprocess
+import shutil
+
+def query(key):
+    if shutil.which("xgetres"):
+        return subprocess.run(["xgetres", key],
+                capture_output=True).stdout.decode("utf-8").strip()
+    else:
+        raise Exception("xgetres must be installed for this theme")
+