* Fixed login --server crash on scheme-less URLs

Necessary to keep `trmnlp login --server` from crashing when given a
scheme-less URL such as localhost:3000, which URI.parse leaves with a
nil host.

The trmnl.com check now lives on Config::App#trmnl_host?, beside the
base_uri it reads, so the command no longer reaches into URI internals
and the nil host is guarded in one place.

Specs drive the subject through a single let(:server), drop an example
that duplicated the existing user_ rejection, and add coverage for the
declined re-authentication and scheme-less server paths.

* Updated version to 0.8.8

Necessary to release self-hosted (BYOS) login support: the `trmnlp
login --server` flag, acceptance of non-trmnl.com token formats, and
listing of plugins with a nil plugin_id.