Support --overwrite options

[tengyifei]

Fixes #39

Problem does not appear on Windows.

Tested on Mac. Would appreciate anyone to try this on linux.

[junosuarez]

@tengyifei could you please rebase and squash into one commit?

[tengyifei]

Done.

[tengyifei]

Any update on when this could get merged?

[kfranqueiro]

It should probably have been noted that this PR never touched the windows platform code path...

I will have this resolved for all 3 centrally in index.js (and in usage.txt and the readme) on the branch that I PR once I figure out rebasing it.

Edit: I just noticed the note in the initial description. I suppose this is only addressing the errors that occur normally if you try to package into an output folder that already exists. In the process of working on --all, though, it made more sense to me to make it explicitly skip or overwrite depending on whether the flag is set. (I could've sworn I saw it cause errors regardless of target platform.)