wishlist: add a "what I plan to do" mode to show what would change

[slippycheeze]

It would be nice to get a preview of what updtr is planning before letting it give it a shot, for my workflow; I can use npm outdated to get what I expect to be the same information, but I have not inspected the code of updtr to verify that this will necessarily always be correct.

I am not thinking "run the tests with the update, but do not commit it", but rather, "show what will be tried." The attempt may, of course, fail, and lead to the normal handling of test failures after update, which may vary from the plan.

I am thinking of a display specifically of something like:

# package.json contains "is-binary-path": "^1.0.0"
] updtr --what-you-planning-machine
is-binary-path@1.0.0 ⃗ is-binary-path@2.0.0
] updtr --what-you-planning-machine --to wanted
is-binary-path@1.0.0 ⃗ is-binary-path@1.0.1

You, uh, might want to find someone more competent at naming things than me to work out what the flag should be called, though. :)

[jhnns]

This should be pretty straight-forward to implement. updtr runs an init task before updating the packages. The init task finishes with a result object that contains all the planned tasks.

The only problem is that init also executes npm install to ensure that all modules are installed before running npm test (#29). It would be necessary to move that part out of init and just before runUpdateTasks.

Would you be open for a PR? :)