Added PdfMerger Writer.

@mvriel Related to #1265 there is a dependency on a missing PDF merging util. So here it is. Template is comming...
Can this method and the autoload_register in line 70 be moved to the Pdf plugin's service provider? This is not something the application needs to load.

Should we move the plugins to separate repos (we are planning to) then this is not necessary in the base package