Allow a custom stdout and stderr

[timstableford]

Expose a way to capture stdout and stderr

[timstableford]

The same is true here as #20

This could overwrite print but the IO library complicates that a little. Perhaps just allowing print to be captured is enough

[timstableford]

#31 makes it easier to overwrite pre-existing library functions

[tims-bsquare]

I'm happy for this to be closed, overwriting the library print has turned out to be enough for me even if not the simplest approach for library usability