Add rebar3-hex plugin

[evnu]

This pull request is a follow-up to #211 and alfert/propcheck#121. The pull request adds the rebar3_hex plugin to allow publishing to hex.pm. If the user is already registered with hex.pm, publishing boils down to two steps:

• rebar3 hex user auth (authenticate with hex.pm)
• rebar3 hex publish (publish to public hex.pm)

Note that compiling with rebar from the repository itself results in a warning, but the build completes:

./rebar compile
WARN:  Missing plugins: [rebar3_hex]
==> proper (compile)
...

@kostis Can you CC the people responsible for publishing to hex.pm? I tried the plugin using a small example project, but did not dare to try rebar3 hex publish within PropEr itself.

[kostis]

Thanks @evnu -- I will merge this.

Regarding the warning on ./rebar compile, I do not worry about it because it's not my intention to be using PropEr's own rebar for compiling from now on. However, ./rebar eunit, which is still used (until I find out a way to shut off the warnings of the test suite when using rebar3 unit), also produces the same warning. Anyway, I hope it's something only temporary.

Regarding notifying the people responsible to hex.pm, I think they get these PR notifications, but I will cc: them anyway (hi @ferd!). However, note that PropEr version 1.4.0 is not ready to be published yet. It will be once you see me changing the badge / README on GitHub.

[ferd]

You actually don't want this plugin, now everyone using PropEr gets to download the hex plugin that way. It should instead go in {project_plugins, [rebar3_hex]} if it is intended to only be used by project owners.

Generally we advise that only project owners use the hex plugin to publish, and that can be done using a global plugin rather than the local one. In any case, we publish the rebar3_hex package with mix so that it is better supported for Elixir folks without having to specify the manager: :rebar3 config value while it still works transparently for users of both rebar3 and erlang.mk.

I can publish new versions of PropEr to hex without a problem (once it is ready), but I would revert this commit first, since it will likely "pollute" builds and not add anything required for users of the library.

[evnu]

@ferd Thanks for clarifying this!

[kostis]

I've created a PR (#214) that reverts this -- also, see a comment there.