Add load average support

[pschmitt]

This PR adds support for load averages via:

• #{load}: Displays load averages (eg: 0.02 1.31 0.78)
• #{load1}: Displays load average of last minute (eg: 0.02)
• #{load5}: Displays load average of last 5 minutes (eg: 1.31)
• #{load15}: Displays load average of last 15 minutes (eg: 0.78)

There's also a new config option: @load_per_cpu_core that when set to true will divide the load average displayed by the number of CPU cores.