add option to skip conflicts when uploading

The option --skip or -s for `kanso upload` makes the command
ignore document update conflicts and report them as "skipped"
While still treating this as success (errlevel 0)

The reason for this modification is to allow for automating our
deployment which ensure the presence of a few documents but
still allows them to be modified later by the application


I could not find how to write or even run tests, so I'm expecting some pointers for that so I can add proper testing to this.