Extend instance configuration with config.d/*.php

Currently all static configuration is loaded only from `config.php`.

However, we commonly have some additions to Observium instance added to config.php. These include, for example, addition of some monitoring plugins not included by Observium Professional by default:

// DNS Probe

$config['probes']['check_dns']['enable'] = 1;

$config['probes']['check_dns']['descr']  = 'Tests DNS';

$config['probes']['check_dns']['args']['default']         = "-s %hostname%";

// NTP Probe

$config['probes']['check_ntp_time']['enable'] = 1;

$config['probes']['check_ntp_time']['descr']  = 'Tests NTP';

$config['probes']['check_ntp_time']['args']['default']         = "-H %hostname%";

Instead of managing these as custom modification (appended to) config.php file, it would be cleaner to have config.d/ directory. Directory can also be named conf.d/ to conform with linux/unix convention. Contents of this directory should be excluded from version control.

Observium would read config.php first and then include all .php files from config.d/*.php. Dotfiles should be ignored and files having non-php extension.

This way Observium users could dynamically, cleanly and bit more safely add instance specific customizations (or any other static configurations) such as

config.d/config.ntp-probe.php

// NTP Probe

$config['probes']['check_ntp_time']['enable'] = 1;

$config['probes']['check_ntp_time']['descr']  = 'Tests NTP';

$config['probes']['check_ntp_time']['args']['default']         = "-H %hostname%";

This feature can currently be implemented as custom "footer code" appended to config.php, but it would nice if unmodified Observium Professional provided this.