Added the from_i3 path source, which calls 'i3 --get-socketpath' as m…

…entioned in the IPC docs.

i3ipc/i3path.py

@@ -1,4 +1,5 @@
 import os
+import subprocess
 
 
 I3_SOCK_ENV_VAR = 'I3SOCK'
@@ -9,7 +10,7 @@ def get():
     """Get the I3 socket path, trying several strategies.
 
     """
-    for source in (from_env, from_x11, from_xdg, default):
+    for source in (from_env, from_x11, from_xdg, from_i3, default):
         try:
             path = source()
             if path is not None and os.path.exists(path):
@@ -100,6 +101,13 @@ def from_xdg():
             return path
 
 
+def from_i3():
+    try:
+        return subprocess.check_output(["i3", "--get-socketpath"])
+    except subprocess.CalledProcessError:
+        return
+
+
 def default():
     """Get the default I3 socket path.