Use migrations/sample-migration.js as sample content if it exists

[tennox]

This PR adds an easy way to provide a sample migration that will be used when the create command is executed.

It checks if the file exists, and if it does - uses that: <migrations-dir>/sample-migration.js

Checklist

• npm test passes and has 100% coverage
• README.md is updated

[coveralls]

Coverage remained the same at 100.0% when pulling 226911a on TeNNoX:override-sample-migration into 9866d65 on seppevs:master.

[coveralls]

Coverage decreased (-0.6%) to 99.448% when pulling 0bffe1b on TeNNoX:override-sample-migration into 296c44d on seppevs:master.

[DanielRuf]

I fail to see the advantage of this.

[tennox]

Well, the advantage is that you can override the default sample migration script.
I (needed to) modify it a bit, added customized examples, also refactored to async functions - I can imagine other use cases.

[seppevs]

Merged and released into migrate-mongo 7.1.0 🎉