Use a specific 'qutebrowser_editor_' prefix for <C-e> instead of 'tmp'.

Why does this matter? In my vimrc I have this:

	" When using dwb <C-e>; assume markdown, and don't store in viminfo since these are
	" temporary files
	autocmd BufRead,BufNewFile /home/martin/.cache/dwb/edit* setlocal ft=markdown viminfo=

I would like to do the same with qutebrowser, but this is not possible with a
file name like '/tmp/tmpSJsgSG4'

qutebrowser/misc/editor.py

@@ -122,7 +122,8 @@ class ExternalEditor(QObject):
             raise ValueError("Already editing a file!")
         self._text = text
         try:
-            self._oshandle, self._filename = tempfile.mkstemp(text=True)
+            self._oshandle, self._filename = tempfile.mkstemp(text=True,
+                    prefix='qutebrowser_editor_')
             if text:
                 encoding = config.get('general', 'editor-encoding')
                 with open(self._filename, 'w', encoding=encoding) as f: