Feature: Suppressable output forwarding

Add '-l' switch to suppress output forwarding

Add an option to disable command output being forwarded to checkrun's stdout.
This is useful for avoiding polluted journals when running systemd services.

Fix: Improved mail header

Makefile: Beautify DESTDIR/PREFIX usage

Avoid triple-slashes in install path.

Feature: Configurable Subject

Change default subject format string

Make mail subject shorter by replacing "(returned %d)" with shorter "[%d]" for
exit code.

First stable release

v1.0

README: Wrap long lines