Add DB URI Options Parameter and env variable

[pkosiec]

Add another parameter which enables user to provide options for DB URI without need to use custom DB_URI.

For example, the following environmental variable should append ?replicaSet=test&connectTimeoutMS=300000 at the end of the constructed DB URI:
DB_OPTIONS='replicaSet=test;connectTimeoutMS=300000'

See #18 for discussion regarding this feature.