Resolve configuration file path on migration

[p-baleine]

It changes so that the path of the migration config file is resolved before it is required.
This avoids Cannot find module error when the configuration file path is specified as relative path:

$ cd some-project/
$ cat db/config.js
// Sample Config File
module.exports = {

  directory: './migrations-folder',

  database: {
    client: 'sqlite3',
    connection: {
      filename: './db.sqlite3'
    }
  }

};
$ ./node_modules/.bin/knex migrate:make create_posts -c ./db/config.js

[tgriesser]

Cool, thanks @p-baleine!