Add trailing newline to last printed message for pg-unfollow

Otherwise, the shell prompt would be printed immediately after the
message.
remove accidental service name from plan name completion list
cleanup addon-add completion
make argument for app flag not optional
Move hk credential and client management to a package: hkclient

When writing hk plugins or a lot of small client software, figuring
out how to get credentials is a burden.

To help with this, move all the credential and HTTP endpoint discovery
and configuration logic into a package that can be imported elsewhere,
with the explicit goal of with having closely-coupled parity with
'hk'.
add import "os" for Windows
goimports
remove obsolete docs
use os/user to determine home directory on Windows
consolidate homePath() logic in hkclient as HomePath()
display of SSL endpoint info, ability to add certs

Added ssl and ssl-cert-add commands.
add ssl-cert-rollback command
cleanup addon-destroy warning
add ssl-destroy command to remove an SSL endpoint
zsh completion for ssl commands

Fixes #143.
complete all files, not just .pem (thanks @dpiddy)
handle the nonexistent netrc case inside hkclient
use fatal errors in auth where appropriate
Only read body for PATCH/PUT/POST.
Add addon-plan command

Enable users to change (upgrade/downgrade) an addon's plan.