Feature pull rebase

[AoDev]

Add the pull --rebase option.

Per git pull docs, you can set the rebase option to true or preserve.
Also requested here: #78

[dylancwood]

Thanks @AoDev
Just to be sure that no one accidentally changes your === to == in the future, I suggest adding the following test:

it('should refuse --rebase option with invalid value', function (done) {
        var options = {
            branch: 'master',
            rebase: 'true'
        };

        new Test(command, options)
            .expect(['pull', 'origin', 'master'])
            .run(done);
    });

Let me know what you think.

[dylancwood]

In fact, it may be prudent to throw an exception inside your customFlagFn if the value of rebase is not true or "preserve", rather than returning a falsy.