generate README and man page from --help

DRY the doc update process and generate the man page
and README using a shell script to combine the EXAMPLES
and help2man to convert the lot to man format.

We generate at dist time so as to avoid pushing the
help2man dependency to packagers.
We checkin the README so that it's easily available
and directly usuable by github.

Note when running `make dist` we should ensure that
docs are updated fully before that is run. I.E. make dist
should not result in any pending git changes, but
if that's the case it should be obvious and can be
addressed before release.
provide alternative --rewrite file editing option

There are enough caveats with the default file replacement method
that it's worth providing in place rewriting as an alternative.
provide --output option to allow redirecting output
use only the base 'crudini' name in --help

On some systems the full path will be in argv[0]
even when crudini is invoked from the $PATH,
so it's better to err on the more concise and
more usually correct basename.