Add infrastructure to print diffs of outputs with a command

[Emilgardis]

A tool to get diffs of output on code changes would be great to have, this would enable something like

@ghost diff all

which outputs

(x) changes have been found in code generation

ARMCM3xxx diff

++++ imagine a diff here  

etc etc

[burrbull]

Looks similar to mmaps. cc @adamgreig