Indicating an ENV variable to use in a specific database.json environment throws an error if not set in current environment

[fklingler]

Following discussion on #391.

With this config file:

{
  "defaultEnv": "development",

  "development": "mongodb://localhost/myproject-dev",
  "production": {"ENV": "MONGO_URL"}
}

If I'm on the development environment, and if the process.env.MONGO_URL variable is not set, it will throw this error:

$ node node_modules/db-migrate/bin/db-migrate up -e dev --config "./database.json" -v
[INFO] Detected and using the projects local version of db-migrate. '/.../node_modules/db-migrate/index.js'
[ERROR] TypeError: Parameter "url" must be a string, not undefined
    at Url.parse (url.js:87:11)
    at Object.urlParse [as parse] (url.js:81:5)
    at module.exports (/.../node_modules/parse-database-url/lib/parse-database-url.js:16:23)
    at Object.exports.loadObject (/.../node_modules/db-migrate/lib/config.js:67:18)
    at Object.exports.loadFile (/.../node_modules/db-migrate/lib/config.js:59:18)
    at loadConfig (/.../node_modules/db-migrate/api.js:555:18)
    at Object.dbmigrate (/.../node_modules/db-migrate/api.js:67:17)
    at Object.module.exports.getInstance (/.../node_modules/db-migrate/index.js:56:10)
    at /.../node_modules/db-migrate/bin/db-migrate:34:23
    at /.../node_modules/resolve/lib/async.js:44:21
    at ondir (/.../node_modules/resolve/lib/async.js:187:31)
    at onex (/.../node_modules/resolve/lib/async.js:93:22)
    at /.../node_modules/resolve/lib/async.js:24:18
    at FSReqWrap.oncomplete (fs.js:123:15)

Apparently, db-migrate iterates over everything (even the non-current environment) in the database.json file, and if there is something in the form of {"ENV": "XXXX"}, it will try to parse process.env.XXXX using parse-database-url.
Since process.env.XXXX is undefined in some cases (potentially all the other environments that are not using {"ENV": "XXXX"} in the config) and parse-database-url doesn't accept undefined, it throws this error.

[wzrdtales]

Also check for the environment variable itself for existence. Skip the environment compeletely if it is undefined. A verbose warning message shall be printed.
https://github.com/db-migrate/node-db-migrate/blob/master/lib/config.js#L90-L91

Also add a verbose warning message to the following part:
https://github.com/db-migrate/node-db-migrate/blob/master/lib/config.js#L98-L99