Adding postgres option to generator

This adds support for a `-d` or `--database` option to the `napa new` generator. Like so:

```
# napa new --help
Usage:
  napa new APP_NAME [APP_PATH]

Options:
  -d, [--database=DATABASE]  # Preconfigure for selected database (options: mysql/postgres/pg)
                             # Default: mysql
```

So, for example:
```
napa new new_service --database=pg
```
Will generate an app using Postgres instead of MySQL.

@jdoconnor @dshemenski @skwp