scripts: (battery) support individual batteries

Optionally use the instance property to match individual batteries,
instead of using the first one. 0 (the default if not specified) is
BAT0, 1 is BAT1, etc.

[vd: use default error message with die() and update the default config]

Closes #60
scripts: (disk) do not discard stderr

In the meantime, comment out the default instance, add a label and
group it with the memory blocks.
manpage: fix a small spelling error

Debian's lintian tool checks for common spelling errors in packages, it
reported this one.

Closes #62
Merge branch 'master' of https://github.com/vivien/i3blocks

Conflicts:
	scripts/battery