fix command line escaping for arguments containing spaces

Hi Pazz,

Here is a tiny pull request for a little issue I encountered when attaching files with spaces in it.
The attach command was rejected with "Unrecognized argument".

After some search, it seems the problem lies in shlex which removes backslashes that are not espaced so I added a line to escape backslashes like wthat was already done for ' and "".

Yann