Auto-boot main app in OTP app project templates

People are starting to expect these niceties! It's also easier to drop
the config when you know it's there than add it when you don't know it
exists.

priv/templates/app.template

@@ -6,7 +6,7 @@
 {template, "app.erl", "{{name}}/src/{{name}}_app.erl"}.
 {template, "sup.erl", "{{name}}/src/{{name}}_sup.erl"}.
 {template, "otp_app.app.src", "{{name}}/src/{{name}}.app.src"}.
-{template, "rebar.config", "{{name}}/rebar.config"}.
+{template, "app_rebar.config", "{{name}}/rebar.config"}.
 {template, "gitignore", "{{name}}/.gitignore"}.
 {template, "LICENSE", "{{name}}/LICENSE"}.
 {template, "README.md", "{{name}}/README.md"}.

priv/templates/app_rebar.config

@@ -0,0 +1,7 @@
+{erl_opts, [debug_info]}.
+{deps, []}.
+
+{shell, [
+  % {config, [{config, "config/sys.config"}]},
+    {apps, [{{name}}]}
+]}.