conf: add config files to module directories

The idea here is that the config files in the module directories will
serve as the canonical documentation of all module configuration
options.

src/mod/asr_tts/mod_cepstral/conf/cepstral.conf.xml

@@ -0,0 +1,12 @@
+<configuration name="cepstral.conf" description="Cepstral TTS configuration">
+  <settings>
+    <!--
+        Possible encodings:
+        * utf-8
+        * us-ascii
+        * iso8859-1 (default)
+        * iso8859-15
+    -->
+    <param name="encoding" value="utf-8"/>
+  </settings>
+</configuration>