Support multiple ssi files

[sni]

It would be good to have multiple ssi files, instead of having everything packed into one file. for example, it should be possile to have:

$> ls -la etc/thruk/ssi/extinfo-header*   
-rw-r--r-- 1 nagios nagios 272 Mar 29 17:45 etc/thruk/ssi/extinfo-header-mycompany.ssi
-rw-r--r-- 1 nagios nagios 375 Mar 29 17:45 etc/thruk/ssi/extinfo-header-pnp.ssi
-rw-r--r-- 1 nagios nagios 787 Oct 13 16:35 etc/thruk/ssi/extinfo-header.ssi
-rw-r--r-- 1 nagios nagios 696 Jul 26  2015 etc/thruk/ssi/extinfo-header.ssi-example

so everything matching extinfo-(header|footer)(|-.*).ssi should be used.

[sni]

implemented.