uptime plugin

This plugin returns the current system uptime either in seconds or in a user-defined format.

Functions:

uptime()	returns the uptime in seconds
uptime(format)	returns the uptime in a specific format

The format can be specified similar to printf(), but only these tokens will be replaced:

Widget Uptime {
    class 'Text'
    expression uptime('%h:%M')
    prefix 'Up '
    width 9
    align 'R'
    update 1000
}