File embeddings: Allow to embed content as well

[MartinKl]

file_nodes.rs provides a tool to embed files as nodes of type file and setting a path. This could be extended to actually embed the content of the file (as an alternative or additionally).

[MartinKl]

This is a more complex issue and should maybe postponed until there is a more clearly defined use case for it. For example, this would require graphANNIS to handle very large annotation values (in its current design) which it probably can, but it's not intended use. Also, there is currently no mechanism to write the contents of a file as CDATA, which could be triggered by hardcoding it, but that again might make exporting the contents painful.