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. Not only that, but when I run `gh submodule add -b foo --reference foo foo/bar foo`, it will work as expected.

However, when you run `git clone -b gh-pages git://github.com/jingweno/gh.git` and `command git clone -b gh-pages git://github.com/jingweno/gh.git`, this is what you get:

```sh
$ git clone -b gh-pages git://github.com/jingweno/gh.git
> Cloning into 'gh'...
> fatal: Remote branch git://github.com/tgkokk/gh-pages.git not found in upstream origin

$ command git clone -b gh-pages git://github.com/jingweno/gh.git gh
> Cloning into 'gh'...
> (works as expected)
```

It also checks for '=', since you can also use `--reference=foo`.