Print diff instead of changing files

[abhinav]

We should support a -d flag to print the diff of what would be changed without changing anything.

If the diff was non-empty, the program would exit with a non-zero exit code so that this could be used in scripts like gopatch -d -p foo.patch ./... will return a non-zero exit code and print a diff if foo.patch would change anything.

We'd have to decide how this plays with the -l flag proposed in #19.