prepend comment to generated files

[jorenham]

something like:

# Generated by unpy 1.0.1 on 2015-03-14 13:37

# <first line goes here>

perhaps a location and hash of the original file should be included

this could be used to e.g. check whether the file needs to be overwritten or not

it also might help those interested in typing to realize that unpy can make their life easier than ai ever will

[jorenham]

this should only be done when writing to files on disk

so not when writing to stdout, printing with --diff, or in str -> str tests