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:

barberousse/src/edit.rs

Lines 52 to 55 in 2f1f81c

	// FIXME this is not secure as the tempfile will be visible in /tmp
	// would be better idea to create a folder in in /dev/shm with a file mode 600
	// so that only the user can edit/see it, then put the file in it
	let mut tf = tempfile::NamedTempFile::new()?;

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