feat(endpoint): Support endpoint subdirectories

[ketiltrout]

... for better organization; see #160 and https://github.com/chime-experiment/ch_ansible/issues/182

This adds support to cocod for grouping endpoints in a full directory tree, rather than forcing them all to be in a single directory. Each endpoint sub-tree may also provide metadata about the tree by putting it in a file called __meta.conf (two underscores). Currently all that's taken from such a file is a description and/or summary for use in the client help strings.

Directories which contain no endpoints are ignored.

In the client, each endpoint tree (other that the top-level tree) is represented by a click.Group subclass called EndpointGroup which will recursively create more EndpointGroups and EndpointCommands as required by the structure given to it.

In redis, we represent nested endpoints by replacing directory separators with underscores, so, e.g. cluster/stop would be cluster_stop in redis. This has the side-effect of meaning we don't allow endpoints named both alpha/beta_gamma and alpha_beta/gamma. The daemon will detect this sort of (almost certainly unlikely) endpoint name clash and exit with an error if encountered.

A redo of #224