Use go modules to vendor the dependencies we need for the writeas-cli build.

this adds a new sub command accounts, which will list all currently
authenticated user accounts grouped by hostname

this adds a new sub command accounts, which will list all currently
authenticated user accounts grouped by hostname

for inclusion of collection in post parameters

this changes imports to use the github import path
github.com/writeas/go-writeas/v2

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

first moving everything out into the base package
should be followed by some refactoring and reorganizing before creating
two different binaries for write.as and writefreely

This adds a go.mod and go.sum to the project.