Accept comma separated converter names, separate Atom from Html and Gemini converters.

Note: atom must be called separately now because of the separation. Example

txt convert -t htm,atom xyz

git-svn-id: file:///srv/svn/repo/kosuzu/trunk@19 eb64cd80-c68d-6f47-b6a3-0ada418499da

cli/conversion.ml

@@ -9,6 +9,6 @@ type t = {
 
 type fn_t = {
 	ext: string;
-	page: t -> Logarion.Text.t -> string;
-	indices: t -> unit;
+	page: (t -> Logarion.Text.t -> string) option;
+	indices: (t -> unit) option;
 }