Add hello world http method

[hanzei]

This PR adds a hello world http method to the plugin. It also, and I think this more important, adds a test for this.

[lieut-data]

@hanzei, thanks for this! Given the broad spectrum of potential "hello world"-style plugins (hooks, API calls, etc.), thus far we have been targeting https://github.com/mattermost/mattermost-plugin-demo for demos of how to actually write (parts of) a plugin.

This repository is primarily meant as a starting point for a "plugin from scratch", with the minimum necessary (build tooling, configuration handling) to remove before writing your own plugin, enabling a fast clone/copy to get a template.

That being said, I think this is the perfect balance between "sample" and "demo", and I'm in favour of adding, especially as it illustrates how to write unit tests for your ensuing plugin with a minimum of code.

Adding @hmhealey for some additional developer review, but otherwise LGTM :)

[hmhealey]

Looks good to me