Deal with dialyzer issues

We end up:
1. making the main interface (`jsx:`) more readable and easier to maintain
(this allows for easier future dropping of the use of `#config{}` for options),
2. fixing some config.-related `-spec(_).`s that weren't using proper elements,
3. having `any()` become a "specific type" - which seems like an improvement,
4. moving stuff like `list()` or `proplists:proplist()` into more appropriate types,
5. making the main config. element (`jsx_config`) "pretty" visible,
6. creating a visible distinction between a module's config. and the generic `jsx_config` elements

rebar.config

@@ -1,2 +1,17 @@
 {edoc_opts, [{preprocess, true}]}.
 {erl_opts, [debug_info]}.
+{dialyzer, [
+    {warnings, [
+        unknown,
+        unmatched_returns,
+        error_handling,
+        underspecs
+    ]}
+]}.
+{profiles, [
+    {test, [
+        {dialyzer, [
+            {plt_extra_apps, [eunit]}
+        ]}
+    ]}
+]}.