Add support for anvironment variables about postgrator-cli

[madflow]

I believe it is already possible to use env variables - when you use a .js file instead of .json:

// postgrator.js
const url = require('url');

let host; let port; let database; let username; let password;
if (process.env.DATABASE_URL) {
    const dbUrl = url.parse(process.env.DATABASE_URL);
    host = dbUrl.hostname;
    port = dbUrl.port;
    database = dbUrl.pathname.replace(/^\//, '');
    [username, password] = dbUrl.auth.split(':');
}

module.exports = {
    migrationDirectory: 'test/migrations',
    driver: 'pg',
    host,
    port,
    database,
    username,
    password,
};

$ postgrator -c postgrator.js 

[perrin4869]

It is probably irrelevant by now, but this kind of usecase is supported in my fork perrin4869/postgrator-cli - see here.

[perrin4869]

Fixed in #30