i3status: Support %frequency for wireless interfaces

i3status: Support %frequency for wireless interfaces

In many public WiFis, the 2.4 GHz wireless band is slow due to
congestion, while there is still plenty of bandwidth available on the
5 GHz area. So when debugging wireless issues it's convenient to have
i3status display the frequency of the access point that the interface is
connected to.

This patch adds support for the %frequency tag for wireless interfaces,
so for example:
    format_up = "WLAN: %essid - %quality / %frequency"
would result in:
    "WLAN: eduroam - 074% / 2.4 GHz"