Followup to #1564 (cli plugins)

From #1564 (comment):

• https://github.com/docker/cli/pull/1564#discussion_r251818285 handling of docker someplugin --version. The --version / -v flag is handled by the plugin root-command, and therefore shows the docker version information (Docker version 19.03.0-dev, build c8f1f1ee). We should fix this to be handled by the plugin.
• Related to the above; consider making --version / -v part of the pluging "API", so that a plugin can advertise its version (both when calling the plugin directly (docker-myplugin --version), and when calling through the docker cli (docker myplugin --version). Perhaps we should add a standard function and/or define a standard format (docker-myplugin version X, commit deadbeef (ACME, Inc.))
• https://github.com/docker/cli/pull/1564#discussion_r251819147 install location for plugins; choose between ~/.docker/plugins or ~/.docker/cli-plugins → See issue #1677 for discussion.
• https://github.com/docker/cli/pull/1564#discussion_r251816130 decide on the output format in docker --help: make plugin-command stand out from built-in commands, or make them appear as regular commands with a non-obtrusive indicator (*) → See PR #1675.
• https://github.com/docker/cli/pull/1564#discussion_r251814998 decide on the location to show version and vendor for plugins in in docker info → See PR #1689.
• https://github.com/docker/cli/pull/1564#discussion_r251890532 small nit: use displayutils.Ellipsis() instead of a custom implementation. Obsoleted by PR https://github.com/docker/cli/pull/1675.
• https://github.com/docker/cli/pull/1564#discussion_r251822916 small nit: hardening config.Path() to disallow directory traversal
• https://github.com/docker/cli/pull/1564#discussion_r251825582 small nit: fix usage output of the sample plugin

Others:

• plugins shuld be able to indicate if they are "management" vs not commands in their metadata and be listed accordingly in e.g. docker help → See PR #1684.
• plugins should have a field in config.json for their use, see Configuration in design → See PR #1652.
• plugins should call back to main cli using dial-stdio to connect to engine. From https://github.com/docker/cli/issues/1534#issuecomment-446206041 → See PR #1654 .

Start with the remaining unchecked items and the linked discussion from PR #1564, focusing on docker someplugin --version, docker-myplugin --version, and the sample plugin's usage output. Done means deciding and implementing the plugin version API and correcting the sample plugin output, while accounting for the related work already moved to issues and pull requests.