Add the previewFeatures configuration option

[mrazauskas]

The previewFeatures configuration option could enable work-in-progress features like .toBeCallable() or .toBeConstructabe() matchers. This way the matchers (and other features) could be published with partial implementation for wider testing / feedback and implemented gradually.

Usage:

{
  "previewFeatures": ["toBeCallable", "toBeConstructabe"]
}