Allow changing side of position (long/short) in a single order

Hello!

I have some algorithms that are always in the market. They are either long or short 100% of the time. If I am currently long, I would like to be able to place a single order to liquidate my position _and_ go short (and vice versa from short to long). Currently, if I try to do this I get the following error

```insufficient qty available for order (requested: 2, available: 1)```

On Slack, @umitanuki said

> yes that’s correct and we are aware some other brokers allow to do so (and we discussed a LOT when designing the current one). there are also people who wants to avoid accidental overbuy/sell. Feel free to make a feature request and we keep refining

I understand some folks don't want to overbuy/sell, and I also see from #85 that some folks want even more error checking when it comes to situations like this. However, would it be possible to add an option to do _no_ checking and to simply allow going from long to short or short to long in a single order?

Without this, I have to submit two orders when going e.g. short to long. I like to submit all of my orders in the morning, right before the market opens. That isn't possible currently if I switch the side of a position, and I have to wait until the market opens to get the second order in.