[engine] Fix interval configuration

Until now, manually specifying an interval did not work, as a cast to
float was missing. Now, it's possible to specify an update interval in
seconds via "-p interval=<interval>"

fixes #54

README.md

@@ -51,6 +51,11 @@ $ ./bumblebee-status -l themes
 $ ./bumblebee-status -l modules
 ```
 
+To change the update interval, use:
+```
+$ ./bumblebee-status -m <list of modules> -p interval=<interval in seconds>
+```
+
 As a simple example, this is what my i3 configuration looks like:
 
 ```