Implement source group.

Fixes #391

lua/cmp/config/sources.lua

@@ -0,0 +1,10 @@
+return function(...)
+  local sources = {}
+  for i, group in ipairs({ unpack(...) }) do
+    for _, source in ipairs(group) do
+      source.group = i
+      table.insert(sources, source);
+    end
+  end
+  return sources
+end