Barest bones of `MelddRepository` unbuilt Gem.

First steps toward Issue #150. New Gem-like dependency in
`lib/meldd_repository` will transplant/replace current `RepositoryBase`
class with `MelddRepository::Base`.

Note that CI-style running of specs is currently clunky; there's no
(apparent) support in RSpec for running app specs using the app's
bundler configuration, followed by running `lib/meldd_repository`'s
specs using *its* bundler configuration (which is different), all in
the same `rake` run. Until/unless that's figured out, CI configuration
needs to be massaged as the component makeup changes. (Running RuboCop
over the entire ecosystem is no problem.)

RSpec: 664+4 examples, 2 failures
RuboCop: 184 files inspected, no offenses detected