[fature-request] Add a restart method about vue-countdown

[fengyuanchen]

You can call the end method first, and then call the start method to restart.

[DannyFeliz]

@fengyuanchen the problem is that I have some actions in the end event and I don't want to be executed at least if finished for real.

[fengyuanchen]

You can also remount the component.

[DannyFeliz]

@fengyuanchen that is not feasible for my use case.

I created a PR adding the restart method.

[xiaozi]

You can call the end method first, and then call the start method to restart.

end method make totalMilliseconds to be zero, then countdown cannot start again using start method.

[fengyuanchen]

Supports this as of v2.1.0.