add option --watch to xst upload

[JoernT]

this would be my most-wanted additional feature to give up on gulp-exist which never really flew for me due to always necessary patches to gulpfile.

[line-o]

I discussed this feature request with @JoernT

It boils down to adding a --watch option to xst upload. After initially uploading the contents of the source to the target
the process does not exit but watches the filesystem for changes. If a change occurs the changed files or folders are uploaded to exist.

Example

xst up ./source-folder /db/temp --watch