Securely store the tempfiles

[zeapo]

The temp file should not be stored in /tmp/ as this is not a good & secure practice when using a shared computer: https://github.com/zeapo/barberousse/blob/2f1f81c2abc3360c44778c0fb58411506577d7f3/src/edit.rs#L52-L55

An idea would be to use /dev/shm, create a temp folder in it with permissions 600 and create the temp files in there.