Added support for diffing specific symbols in .o files

[Adubbz]

As per title

[simonlindholm]

Sorry for being enormously late to get to this. Isn't the parameter you'd pass to this always the same as the function name you already pass in, so now you'd pass it twice? Feels like there's a lot of overlap with the -s flag, too. (But a more reliable -s that's based on symbol boundaries would be a good thing.)