Add a way of choosing destination file for attachments

[Tristan02d]

When using mu4e-view-save-attachments, only the destination directory can be
set. Therefore if the attachment is Very Inconvenient filename-1234.pdf, it
will be saved in a file with the same name.

At first I thought about creating a new custom variable to tweak the behavior of
mu4e-view-save-attachments--which explains this poor branch name. But I
thought adding a dedicated command makes more sense. One command that does
everything is more likely to become a big piece of inedible code, especially if
I am the guy who writes it. So here is my attempt to implement a facility for
choosing the destination file of an attachment.

I know that this kind of feature requires being properly documented in the
manual, and that binding it to a key would be nice--I think of o for "one" or
"output". I am totally willing to do it, but I would like to get some feedback
on the idea first.