Change the behavior of options.overwrite

[sumeetattree]

Ideally I would like control of what happens when a destination file already exists. Currently the behavior is completely halt the process. I think we need a skip behavior as well.

Proposal: Make the overwrite accept string parameters instead of boolean

This way I can just do overwrite: 'skip' and it'll skip the current upload/download and not halt.

I also suggest the following options:

overwrite: 'skip' // skip the upload, log a warning
overwrite: 'force' // log a warning, overwrite the destination file
overwrite: 'halt' // same as current functionality. halts the process completely.

Would you be interested in a pull request?