Add 'd.eta.*' commands

[chros73]

Add d.eta.* commands:

• d.eta.seconds : estimated time left in seconds
  • 0 : download is partially done
  • -1: download speed is less than 512 Byte/s
• d.eta.time : estimated time left in short human readable format (e.g.: 1h34’)

Use print_download_time_left method from rtorrent as reference.
Wished by @pyroscope.

Usage: eta/last_xfer column