temp: add a "display_if" setting

Adds a "display_if" setting to temp module.  This is a snippet that will
be evaulated, and if the result is true, it will display the module's
output.

i3pystatus/temp.py

@@ -11,6 +11,7 @@ class Temperature(IntervalModule):
     settings = (
         ("format",
          "format string used for output. {temp} is the temperature in degrees celsius"),
+        ('display_if', 'snippet that gets evaluated. if true, displays the module output'),
         "color",
         "file",
         "alert_temp",
@@ -21,11 +22,13 @@ class Temperature(IntervalModule):
     file = "/sys/class/thermal/thermal_zone0/temp"
     alert_temp = 90
     alert_color = "#FF0000"
+    display_if = 'True'
 
     def run(self):
         with open(self.file, "r") as f:
             temp = float(f.read().strip()) / 1000
 
+        if eval(self.display_if):
             self.output = {
                 "full_text": self.format.format(temp=temp),
                 "color": self.color if temp < self.alert_temp else self.alert_color,