Introduced a start and stop function

[legovaer]

This PR introduces two new commands: start and stop. This will allow a user to start analyzing all executed PHP scripts between the start and the stop command. Every PHP file which needs to be analyzed needs to include the autocoverage PHP file:

require <phpcov_directory>/src/lib/autocoverage.php

Then a coverage analysis can be started by executing:

phpcov start

As soon as all php files have been executed, the coverage then can be stopped by executing

phpcov stop

Which accepts exactly the same arguments / options as phpcov execute

[sebastianbergmann]

This is outside the scope of this project, sorry.