Programmable API createDatabase arguments are overridden

## I'm submitting a...

- [X] Bug report
- [ ] Feature request
- [ ] Question

## Current behavior

Command line parameters override `createDatabase` parameters in [Programable API](https://db-migrate.readthedocs.io/en/latest/API/programable/#createdatabasedbname-callback).

```
$ DATABASE_URL=postgres://localhost/postgres node index.js
[INFO] Created database "expected"
$ DATABASE_URL=postgres://localhost/postgres node index.js unexpected
[INFO] Created database "unexpected"
```

## Expected behavior

I would expect parameters provided to `createDatabase` always override possible command line arguments.

```
$ DATABASE_URL=postgres://localhost/postgres node index.js
[INFO] Created database "expected"
$ DATABASE_URL=postgres://localhost/postgres node index.js unexpected
[INFO] Created database "expected"
```

## Minimal reproduction of the problem with instructions

```
$ cat index.js
var dbmigrate = require('db-migrate');
var dbm = dbmigrate.getInstance(true);
dbm.createDatabase('expected')
.then(function() {
process.exit(0);
});
```

## What is the motivation / use case for changing the behavior?

Using `createDatabase` through Programable API in an program that has command line args is not possible at the moment.

## Environment

db-migrate version: 0.11.1

db-migrate-pg version: 0.4.0


Additional information:

- Node version: v10.6.0

- Platform:  Mac

---
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