File input & output

stdin and stdout support is already implemented, however it would be useful to input from named files and output to named files.

Input should support multiple files to read from, they will all be in the same format and added in at the same point in the pipeline.

Output should only support a single file, will output one row from the buffer at a time.

Because input and output require parameters they must parse the string provided at module initialisation.

These should be something like:

file.in=/file1:/file2:/file3

What should the behaviour be if one of the files specified can't be opened?
- Should it fail if any can't be opened?
- Should it fail if none can be opened?
- Should it try and re-open files periodically
- Should it have a write buffer with periodic flushing?