Add a a pre-install config

[svedova]

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

Users who'd like to configure private registries may need to configure yarn and npm. Since Stormkit automatically calls yarn or npm install it's not possible to do so today.

With npm it's possible to use the NPM_CONFIG_REGISTRY environment variable, however with yarn this doesn't seem possible.

Describe the solution you'd like

A section in the build config to configure pre-install scripts.

[svedova]

We plan to introduce an install field where users can specify the install command. It should also satisfy this feature request.