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
Merge pull request #413 from Floby/upload-skip-conflicts

add option to skip conflicts when uploading.