Add sharing via Twitter Direct Message (DM)

Closes #373

Twitter gets points for making this as easy as it should be. In order for the
access token to get DM access, the developer must include `?flag=dm` during
authorization (will be documented).

I put this at `POST /types/messages`, which will be the endpoint for LinkedIn as
well, and where we can also put a consolidated inbox as we support reading too.
Factor out LinkedIn status update function

Just to keep the exports easy to read. Maybe we can reuse some of it, but a lot
is already in the `linkedin` library.
Add sharing via LinkedIn Messages

Closes #744

Pretty straightforward, just use
- to: user@linkedin
- title: Subject line
- body: Contents

Recipients must be referenced by their per-application generated ID; not their
numeric LinkedIn ID, nor the name in the custom URL. Will document.

Since LinkedIn response with absolutely nothing besides an `HTTP 201`, I've
inserted a friendly `success: true` to let developers know all is well.
Merge pull request #750 from Singly/messages

Sharing via Messages