Implemented module support
* module support aims to enable extraction of core static site generation functionality implemented for my personal blog into a separate project
** i.e. the detail transformations of repository [blog.kummerlaender.eu](https://github.com/KnairdA/blog.kummerlaender.eu/)
* a BuildXSLT module is a simple XML file definining the transformations which make up the module
* modules may be built using the newly implemented "module" task type
** this task requires a input and definition node
*** the input node is resolved in the same way as "generate" input nodes
*** the definition node currently only supports the file mode
** chained transformation links inside the definition are resolved relative to the definition files location
*** this was enabled by commit `a4a7ee4` to InputXSLT
* restructured task processing
* transformations are now read by "InputXSLT:generate" instead of by "InputXSLT:read-file"
** this enables entities to be resolved against the transformation location
** this was enabled through filesystem context resolution changes in commit `3c166ed` to InputXSLT
* added basic error handling based on terminating XSLT messages