Plyr 1.8.1

• New parameter .id to ldply() and rdply() that specifies the name of
  the index column. (Thanks to Kirill Müller, #107, #140, #142)
• New parameter .id to rdply() that specifies the name of the index
  column. (Thanks to Kirill Müller, #142)
• The .id column in ldply() is generated as a factor to preserve
  the sort order, but only if the new .id parameter is set. (Thanks to Kirill
  Müller, #137)
• rbind.fill now silently drops NULL inputs (#138)
• rbind.fill avoids array copying which had produced quadratic time
  complexity. *dply of large numbers of groups should be faster.
  (Contributed by Peter Meilstrup)
• rbind.fill handles non-numeric matrix columns (i.e. factor arrays,
  character arrays, list arrays); also arrays with more than 2
  dimensions can be used. Dimnames of array columns are now preserved.
  (Contributed by Peter Meilstrup)
• rbind.fill(x,y) converts factor columns of Y to character when
  columns of X are character. join(x,y) and match_df(x,y) now work
  when the key column in X is character and Y is factor. (Contributed
  by Peter Meilstrup)
• Fix faulty array allocation which caused problems when using split_indices
  with large (> 2^24) vectors. (Fixes #131)
• list_to_array() incorrectly determined dimensions if column of labels
  contained any missing values (#169).
• r*ply expression is evaluated exactly .n times, evaluation results are
  consistent with side effects. (#158, thanks to Kirill Müller)