Memorise operations

[eliocamp]

Have an option to memorise operations.

If a file exists, then rcdo should try to guess if it's a valid result of the operation. Save hash or timestamp of input files and command line text. If it's valid, return the existing file without re-running the operation.

This will not be so convenient for tempfiles, so add an option to set the tempfile location and the name of the tempfile should be deterministic; maybe based on hash.