Feature/temp

Implementing a temperature monitoring feature as suggested through the issue #139.

The displayed temp is the average of all the temperature that can be retrieve from the sensors.

As I also noted in the issue, it works through the ```sensors``` command which might not be available in every system supported by liquidprompt. The ```acpi``` command has been suggested by @jab416171 but I couldn't make it work.

So, here's the pull request.