Support for connection string format

[YBogomolov]

Driver: PG Steps to reproduce:

.migrorc file with the following content:

{
  "database": {
    "main": "postgresql://user:pwd@localhost:5432/db_name"
  }
}

Run migro up main.

Result:

1. No error messages.
2. No successful/failed migration reports.

Expected result:

1. Either error message or migration report.

[YBogomolov]

Not a bug. To use connection strings with PostgreSQL, one needs to write the config as following:

{
  "database": {
    "main": {
      "connectionString": "postgresql://user:pwd@localhost:5432/db_name"
    }
  }
}