Add shell module

Add new "shell" module which can execute arbitrary shell command and display
its output.

i3pystatus/shell.py

@@ -0,0 +1,36 @@
+from i3pystatus import IntervalModule
+from subprocess import check_output, CalledProcessError
+
+class Shell(IntervalModule):
+    """
+    Shows output of shell command
+    """
+
+    color = "#FFFFFF"
+    error_color = "#FF0000"
+
+    settings = (
+        ("command", "command to be executed"),
+        ("color", "standard color"),
+        ("error_color", "color to use when non zero exit code is returned")
+    )
+
+    required = ("command",)
+
+    def run(self):
+        try:
+            out = check_output(self.command, shell=True)
+            color = self.color
+        except CalledProcessError as e:
+            out = e.output
+            color = self.error_color
+
+        out = out.decode("UTF-8").replace("\n", " ")
+
+        if out[-1] == " ":
+            out = out[:-1]
+
+        self.output = {
+            "full_text": out,
+            "color": color
+        }