Add head() method to get first N messages in sequence

src/imapdomo.lua

     return result
 end
 
+pkg.head = function(num, seq)
+    --
+    -- Return first *num* elements from sequence
+    --
+    local result = seq:is_smaller(0)    -- HACK to generate empty sequence
+    for idx, value in ipairs(seq) do
+        if idx > num then break end
+        table.insert(result, value)
+    end
+    return result
+end
+
 pkg.filter_part_like = function(query, seq)
     --
     -- Run MIME part query on *seq* sequence of messages