showlinenum : Add options, usage and examples

- Add command line options.

The output format can now be controlled by using command line options.
git diff [options] <required> | showlinenum.awk [option=<value>]

@show_header [0,1] default: 1
Show diff headers.

@show_hunk [0,1] default: ( show_header ? 1 : 0 )
Show line hunks.

@show_path [0,1] default: ( show_header ? 0 : 1 )
Show paths before line numbers.

- Change the default output format to show diff headers and prepend
line numbers only.

Prior to this change both the path and line number were prepended and
the diff header wasn't shown in full.

- Use an initialization routine instead of BEGIN.

The initialization routine is called once after the first line is read
but before it's processed. Initializing this way ensures the options
passed on the command line are parsed regardless of whether they're
passed using awk's -v option or the traditional way.