Resolves T604 Fetch users posts if authenticated

This changes the 'posts' subcommand, previously 'list', to also list any
remote posts for the user if currently authenticated.

• adds DoFetchPosts to the api pkg which returns all the logged in users

posts.

• adds a RemotePost type to the api pkg which stores some of the return

information from DoFetchPosts for use in printing to stdout

• adds GetUserPosts to the api pkg which converts the incoming

writeas.Posts into the smaller RemotePost type

• adds getExcerpt which takes the incoming writeas.Post.Content and

returns a trimmed down excerpt of the content. Max 2 lines of 80 chars
and delimited by '...'

• changes CmdList from pkg commands to CmdListPosts which now writes out

any local and remote posts in simple formatted tables