Replace - with _ when user passes "extra_arguments"

[lorengordon]

When using an explicity-defined cli opt, argparse replaces - with _ by default. We are also accepting arbitrary cli opts into the extra_arguments list, which argparse does not munge. For the user, this means currently they would need to do this:

watchmaker --content_source <url.zip>

which doesn't match the structure of every other cli opt. Instead, we should munge extra_arguments to support the same structure as regular cli opts:

watchmaker --content-source <url.zip>