Add Attachment::download{,to_directory}
Add Attachment::download{,to_directory}

Adds two methods to the Attachment model:

- download: uses Hyper to download the attachment and return it as a vec
  of bytes;
- download_to_directory: equivilant to `download`, except it will also
  save the bytes to a file named equivilant to the filename in a given
  directory.

Check the documentation for Attachment for more information and
examples:

<https://docs.austinhellyer.me/serenity.rs/latest/serenity/model/struct.Attachment.html>

acdenisSK/serenity,Flat/serenity,Roughsketch/serenity,zeyla/serenity.rs,zeyla/serenity,eLunate/serenity,Lakelezz/serenity