Add ability to set transferred file name

As per a private conversation with an user, it would be useful to have the possibility of specifying the name of the transferred file.

For a single file:
```
qrcp send MyFile.pdf --name attachment.pdf
```
For multiple files:

```
qrcp send MyFile1.pdf MyFile2.pdf --name attachment.zip
```

Open questions:
- should the file extension be automatically added?
