Add the documentation with the example how to use _run function

_run function (see below) is not documented (https://db-migrate.readthedocs.io/en/latest/API/programable/#run)

/**
* Gets a connection and runs a mongo command and returns the results
*
* @param command - The command to run against mongo
* @param collection - The collection to run the command on
* @param options - An object of options to be used based on the command
* @param callback - A callback to return the results
*/
_run: function(command, collection, options, callback) {

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