Added "gh submodule add"

I added `gh submodule add` command.

The `continueNext` variable is used to skip options **and** their arguments, i.e. you can skip `-b`, `--reference`,  etc. wherever they are. So, when I run `gh submodule add -b foo --reference foo foo/bar foo`, it will work as expected.

It also checks for '=', since you can also use `--reference=foo`.