Add ability to backup only single database

[vdmitriy]

Hi @istepanov. Thank you for great tool. Please merge my improvement to backup only single database.

[tenorok]

It seems that this repository is orphaned...

Opportunity to backup only single database is good, but what if I need to backup two databases? Unfortunately now mongodump not providing opportunity to backups multiple databases at one time (TOOLS-22, TOOLS-31).

But in my fork I'm created environment variable MONGO_DB_NAMES, which workaround this injustice. For dump selective databases specify they names through space. This will create few archives with names like 20170218_025718-db1 and 20170218_025718-db2 for each database from option.