Makefile: add a distclean target

i3blocks.1 is removed during a "make clean", which is not convenient
when used from an archive. Create a "distclean" target for this purpose.

Reported-by: Jason Pleau <jason@jpleau.ca>
Makefile: compress dist archive

The generated archive was not gzipped. Add a -z flag to the tar command.

Reported-by: Jason Pleau <jason@jpleau.ca>
config: remove useless battery argument

Refs #53
scripts: (volume) make it more flexible

- Dynamically choose main volume control
- Selectable volume adjustment step
- Selectable volume scale (percent/decibels)
- Allow capture devices
- Better option documentation

Refs #53
scripts: (memory) use instance

Refs #53
scripts: (disk) use instance

Refs #53
prepare 1.3
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