Add full magnet link support

[chros73]

Add full magnet link support:

• add cfg.magnet_link.queue variable:
  • enabled by default
  • whether to queue a magnet link download upon initial downloading to be able to use Queue Manager instead of start downloading it straight away
• modify addMagnetLink.sh external script (rename it to addMagnetLinksNative.sh) to be able to:
  • accept multiple magnet URIs
  • use in scripting
• add alternative addMagnetLinksAria2.sh external script if the above fails:
  • it downloads and creates torrent files from magnet URIs into one of the "queue" (!) directories of rTorrent
    • so it doesn't take into account the cfg.magnet_link.queue variable
  • accept multiple magnet URIs
  • use in scripting
  • requires aria2c command

Note that the native way is the preferred way:

• downloaded torrent (meta) files can't be renamed by aria2.

Also update corresponding WIKI page.

Refers to: #74 , #102 , #117