Make the --force a real flag

[AtomicVar]

Make the "--force" a real flag that does not require anything after it. Fix #690

The old code:

if (!args.force && helpers.model.modelFileExists(modelPath)) {

make the sequelize ... --force useless because args.force is a '' which yields !args.force to be true. And the system will ask the user to add --force again. That's what I encountered in #690.

In a nutshell, we need to add something after the --force to make it work in this old situation.

Now it is:

if (args.force === undefined && helpers.model.modelFileExists(modelPath)) {

so when the user adds --force, the args.force is not undefined, so everything is OK.

Test

I compiled it and run npm test and it passed.