Created three individual Strings for each battery status (charging, dicharching, full)

Replaced hard coded status strings (CHR, BAT, FULL) in
print_battery_info.c with user defined strings. The new strings are
'status_chr', 'status_bat' and 'status_full' and can be set in i3status.conf.
e.g.
status_chr = "⚡ CHR"

If any of the new status strings is omitted the standard strings (CHR,
BAT, FULL) are used.
Updated the man page for i3status with information about the new individual strings that can be used for each battery status.
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"
support the special interface name _first_ for ethernet/wireless

Since we have deterministic device names in Linux, these strings are a
much better default in the i3status config than "eth0" and "wlan0" (what
we used before).
Allocate and expose per-instance pointers for plugins
Avoid use of uninitialized variable in print_battery_info

The boolean flag 'watt_as_unit' may be used without being initialized if the
configured battery path does not contain expected statistics (for example if
it is misconfigured and points to AC adapter info or simply an unrelated file).

Even though it does not cause ill effects, it causes a warning (true positive)
when running i3status under Valgrind.  Initialize the variable to make code
well-defined.
make github the canonical i3status repository