This commit also includes a way to completely disable Eunit
as that is generally desirable for Elixir-only projects.

This allows overriding the default which is to use the same
explicit version as the dep_* line, and instead specify
ranges of versions.

We don't need the tarball in that scenario.

Templates now no longer use Make variables for substitution
but instead replace strings with their equivalent:

  template_name: Corresponds to n=template_name
  project_name:  Corresponds to $(PROJECT) or in=project_name

This allows defining templates outside of Makefiles. For
example an external plugin could define their templates
in templates/my_template.erl and then have the following
in the included Makefile:

  tpl_my_template = $(file < $(THIS)/templates/my_template.erl)

By default the created file will be in src/template_name.erl.
This can be overriden with the tplp_* variable:

  tplp_my_template = src/model/my_template.erl

Substitution is applied both to the template contents and
to its path.

In addition, attempting to overwrite an existing file when
creating a template will result in failure.

We use $1 instead of $(1) or $p instead of $(p) when possible,
except when building variable names (pkg_$(p)_name for example,
but also $(dep_fetch_$(1))).

This should both be more solid and more secure.