On logout, this deletes host / user directories if they're empty.

Ref T586

Now when running `wf auth`, we'll automatically set the given host +
username as the default account in .writefreely/config.ini, so subsequent
requests without an explicit --host and --user will use this account.

Ref T635 T586

config.LoadUser was not ensuring the directory exists before trying to
load any already authenticated user, fixed using config.DirMustExist

there was a bug where a default user at the config directory root was
not being used for calls not including a flag or host level config.ini

only on wf, writeas still stores in root user config directory

full support, auth and actions working by use of flags or defaults
maintains backwards compatibility with write.as

- now allows authentication with more than one user per host, stored as
username.json inside the [host] directory.
- supports a default user and host in config.ini
- global flags will override the default

- adds new user/u flag to wf
- load and save user file based on username
- removed host flag from writeas
- adds hidden global flag for user to writeas to maintain
compatibility

only if host flag supplied for writefreely binary, writeas unaffected

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

this splits everything out into shared packages

- user config and application config share a package for now
- sync is part of the api package which includes the client, posts and
tor logic
- logging is it's own package
- commands are in their own package

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