Add prices for preemptible instances

[gdubicki]

Is your feature request related to a problem? Please describe.

I want to compare E2 vs N2D preemptible instances across a few regions using your tool and I can't because it doesn't include preemptible prices at all.

Describe the solution you'd like

Add a column within the "Costs" group with the preemptible price.

Describe alternatives you've considered

Using an alternative solution - https://gcpinstances.doit-intl.com - but it has its own issue with no guarantee of data being up to date (see doitintl/gcpinstances.info#34 for more info).

[Cyclenerd]

Currently you can only filter if the instance (machine type) is available as spot (Spot Provisioning Mode). By the way, prices are calculated automatically with this tool: https://github.com/Cyclenerd/google-cloud-pricing-cost-calculator

For Spot there is already a little bit prepared but not yet implemented.

https://github.com/Cyclenerd/google-cloud-pricing-cost-calculator/blob/master/build/mapping.csv#L176

Pull requests are welcome. I don't use spot VMs so I can't control the real calculation that is later on the invoice.

[Cyclenerd]

Prices for Spot VMs are now displayed.