Add support for globally defined forced $CFG

There are some additional $CFG variables that the mdk user may want to always
set in all config.php created. In my case, it is $CFG->divertallemailsto to
prevent accidental mails sent out from my local installations.

The patch adds support for the new mdk-config items grouped under `forceCfg`.
If the setting forceCfg.foobar is found, it's value will be put into
$CFG->foobar right after the installation process.
PHPUnit command does not set --run automatically
Moving the PHP Unit code to its own class

Hopefully not creating billions of regressions...
New directory to store extra web accessible things of an instance
Option to generate code coverage with PHPUnit
Adding missing --symlink from doctor bash completion
Abstracting the methods to get URL or extra dir
Deprecating old way of initialising PHPUnit
Behat command outputs more information to the extra directory
Merge branch 'forcecfg' of https://github.com/mudrd8mz/mdk
Adding forceCfg example to config-dist.json