Support writable stream

[frankleng]

Is your feature request related to a problem? Please describe.
Saw that support for writable streams was dropped, but open to requests.
Our ETL stack depends on writable streams, especially transform streams before writing to rethinkdb.

Describe the solution you'd like
A method similar to rethinkdbdashtoStream({writable: true}), perhaps getWritable()?

Describe alternatives you've considered
Instantiate a writable stream to capture data, then call r.table.insert
It is workable, but thought having driver support for writable stream will make this a lot cleaner.

Appreciate the hard work! This driver has allowed us to revamp and save a lot of code as we move to TS.

[atassis]

I'll think about it, but want to clean and stabilize the code we already have to make it more solid. btw, you can check next version of a package