Add mongodb persistence

[ikatun]

Added --state-db option which contains the name of env variable where DB connection string is stored.

For example --state-db MONGO_DB_CONNECTION_STRING means that connection string for DB is stored in process.env.MONGO_DB_CONNECTION_STRING.

Added a wildcard dependancy for promised-mongo because any version of promised-mongo contains the functionality required for this PR.

Connection string is deliberately not read from the argument directly because usually server environments pass connection string as an env var.

[wesleytodd]

Hey @ikatun! Thanks for the PR, but I do not think we will be moving in the direction of direct support for certain persistence layers.

If you take a look at the work being done on the upcoming 1.x release [1], you can see that "storage engines" is already implemented. All you need to do is provide a compatible "store" [2]. If you want, you can install the fork I was running and get a "work in progress" version of these changes [3].

[1] #77
[2] https://github.com/tj/node-migrate/blob/1.x/lib/file-store.js
[3] https://www.npmjs.com/package/@wesleytodd/migrate

[ikatun]

You could reference the fork of your library (https://www.npmjs.com/package/migrate-mongodb) for all of those poor developers who want to use your library and store the statefile to the mongo database (while we're waiting for that new storage engine/mongodb refactor thingy) :)