Add support for custom _replicator id

[jvdweerthof]

couch-replicator-api.js:

• Add doc_id parameter to CouchReplicatorApi constructor (May be omitted for backward compatibility)
• Add automatic selection of doc_id or db to get, put, del functions (doc_id takes preference over db)
• Add JSDoc for CouchReplicatorApi constructor

status.js:

• Add doc_id parameter to status function (May be omitted for backward compatibility)
• Change filter to match on doc_id if set (instead of target)

Example usage:

const CouchReplicator = require('couch-replicator-api')

    , replicator = new CouchReplicator(
          'http://npm.nodejs.org.au:5984'
        , 'user'
        , 'pass'
        , 'registry',
        , 'registry_to_127.0.0.1:5984'
      )

[jvdweerthof]

In our projects we often use replication id's/names that are different from the database name, mainly to identify to which couch server the db is replicated. I'm aware that we are not alone in this, so this change should benefit a wider audience than just our project.