Make the MongoMigrate class more flexible

[ThibaultLemaire]

We are trying to use pymongo-migrate in a project using mongo-engine, unfortunately your MongoMigrate class doesn't really play nice with that.

After taking a look at the source, I noticed MongoMigrate only really uses a database. The client passed for initialisation is only used to get the default database.

In my case, the client I can get from mongoengine with get_connection() doesn't have a default database and the code raises an exception. However I can get the database I want with get_db().

So I thought MongoMigrate should rather take the db as a constructor argument and maybe provide a classmethod to init from a pymongo.MongoClient.

I can work on a PR if you're okay with that.

[rooterkyberian]

Sounds good, especially since client can be still taken from db.client

maybe provide a classmethod to init from a pymongo.MongoClient.

doesn't sound to be necessary, since that would be just one line