Add additional blockdiag output file format in diagrammer

[LoLei]

The method renderToFile has it's output file format hardcoded as "png". Blockdiag also supports PDF, therefore it could be a valid argument for this function.

This pull request adds a parameter for this, and executes blockdiag dynamically with it.

The argument is not used outside the scope of this function, as I'm not aware of the exact context in which it is executed, as it's been a long time since I've used this program, but I remember wanting the option back then.