Print namespaced commands in help

Prints this for rebar3, for example. `default` namespace is displayed first without the namespace, others come after in alphabetical order and a left margin:

```
as              Higher order provider for running multiple tasks in a sequence as a certain profiles.
clean           Remove compiled beam files from apps.
compile         Compile apps .app.src and .erl files.
ct              Run Common Tests
dialyzer        Run the Dialyzer analyzer on the project.
do              Higher order provider for running multiple tasks in a sequence.
eunit           Run eunit tests on project apps.
help            Display a list of tasks or help for a given task or subtask.
new             Create new project from templates.
pkgs            List available packages.
release         Build release of project.
shell           Run shell with project apps and deps in path.
tar             Tar archive of release built of project.
update          Update package index.
upgrade         Upgrade dependency.
version         Print version for rebar and current Erlang.

efene
  compile        efene rebar3 plugin

erlydtl
  compile        Compile erlydtl templates.
```