mark 0.8.1

• write_file_md5() now supports "feather" and "parquet" methods as wrappers for {arrow} #234
• md5() added to provide MD5 check sums for objects #233
• unique_rows() added to subset on (non-)duplicated rows in a data.frame #87
• within() added as an alternative to between_more() #120
• test updated for upcoming R release #240