Implemented external "generate" function
* this function essentially provides the same functionality as FunctionTransform with the addition of directly committing the result to the filesystem
** this is needed if one wants to generate doctype-containing transformations from inside a transformation as "write-file" is not able to infer the doctype of a given xalan::XalanNode pointer
** this is a integral limitation of (at least) XSLT 1.0 as implemented in Apache Xalan
*** a document can of course only have a single document type
*** e.g. we can not include a doctype node in the result tree of FunctionTransform
* implemented test case for FunctionGenerate
* transformation "test.xsl" is shared between the FunctionTransform and FunctionGenerate test cases
* modified README.md accordingly