Added migrationsList helper to easily build a valid list of migrations

[nicolas-van]

Hello,

This PR brings a solution to a problem that has already been reported, notably in this issue: #184 . This older PR also attempts to bring a solution for that: #188 .

This issue is that the documentation specifies that it is possible to avoid using an automatic file loader for migrations and to specify a list of migrations instead. (Notable use case where we need to do this is when we use Webpack, where a file loader usually won't work as expected.) Unfortunately this is not particularly easy to do. The Migration class is not exposed in the index.js file and, even by importing it anyway, it's still not easy to use because it is very oriented toward that migration-file-loading behavior.

This PR simply adds an utility function named migrationsList() that allow to specify a list of migrations this way:

const umzug = new Umzug({
  migrations: Umzug.migrationsList([
    {
      name: '00-first-migration',
      up: ...,
      down: ...
    }
  ], 
  // a facultative list of parameters that will be sent to the `up` and `down` functions
  [sequelize.getQueryInterface()])
})

I added a test and documented the feature.

[nicolas-van]

Thank you.