Support fetching authenticated user's posts

[thebaer]

Overview
Users should be able to see a list of anonymous posts owned by their account.

Background
Since account posts aren't stored locally, users will need to be able to fetch a list of posts on their account, so they can access the IDs of published posts to update or delete them with the CLI.

Dependencies
User authentication (#10).

TODO: Add a method to the go-writeas library to retrieve an authenticated user's posts. (issue: writeas/go-writeas#5)

Implementation

• When running writeas list, show these remote posts if a user is authenticated

The posts list should also be a little more useful by including post title / content, potentially behind a new CLI flag. Some ideas:

bxp4el2zng8h9    A Post Title
uy9f77u0v7o7d    This is the starting text of my post, truncated somewhere...

We could potentially include unsynced posts in the list, like on the web.

bxp4el2zng8h9    A Post Title
uy9f77u0v7o7d    This is the starting text of my post, truncated somewhere...
UNSYNCED
13nbyi9j3hu1z    Older Post

[thebaer]

Closed by #29