Allow custom info columns in counts table.

libcodonusage/libcodonusage.py

@@ -280,11 +280,13 @@ might be excluded.
 
 def make_counts_only(
         counts_table,
-        seq_id_kw="locus_tag", alt_tag_kw="old_locus_tag"):
+        seq_id_kw="locus_tag", alt_tag_kw="old_locus_tag",
+        ref_info_cols=None):
     """
     Integrate "informative" columns of *counts_table* into the index.
     """
     # To ensure a stable order:
+    if ref_info_cols is None:
         ref_info_cols = [
             alt_tag_kw, seq_id_kw,
             "start", "end", "length",