Support base structure for documents inserted into the DB

[sitegui]

Most documents inserted into a given collection have the same base structure (like Schema in mongoose).

By now, the test writer must explicitly declare all those default values and this creates a lots of repeating boring boilerplate, like:

### order in Order
items: [] // <- boilerplate
totalPrice: 0 // <- boring...
isActive: true // <- ZzZZzz
client:
    name: 'Guilherme' // <- this is useful

It would be nice not to have to repeat myself in those cases 😜

[sitegui]

My proposal is:

Add a new option called defaultDocuments that is a map from collection name to base document. An example:

require('api-test')('test/api-test', {
    // other options here
    defaultDocuments: {
        Order: { // the collection name is the key
            items: [],
            totalPrice: 0,
            IsActive: true
        }
    }
})

Every DB insertion would mixin with the base document (if present).

[sitegui]

It would also be nice to expose the base document in the context, so the test writer can use it explicitly somewhere else than DB insertion.

An example:

### user is
    defaultDocuments.User with name: randomStr()