Make newline-handling more flexible (per WH), declare keywords.

Followed Wes Hardaker's suggestion to make the translation of
newlines more flexible --- instead of making a boolean for
special translation of blank lines, I added the ability to
specify the translation.

Also added a macro for declaring generic translation keywords
with type information and documentation.  Hope this will make
the generic translator easier to use.

contrib/lisp/org-export-generic.el

@@ -465,7 +465,7 @@ newlines are interpreted as significant \(e.g., as indicating
 preformatted text\).  A common non-nil value for this keyword
 is \"\\n\".  Should typically be combined with a value for 
 :body-line-format that does NOT end with a newline."
-    :type string))
+    :type string)