Make commands join args with spaces

This patch ensures the following commands join their arguments with
spaces to make it easier to interact with:

- cf
- mkdir
- cd
- attach
- detach
- ct
- copy
- move
- save

all: purge redundant underscores

Signed-off-by: Wagner Riffel <wgrriffel@gmail.com>

Add filesystem completions for :attach and :cd

Tab-completions now cycle through filesystem paths when using :attach or
:cd commands.

Add :attach command for compose

Allow users to add attachments to emails in the Compose view. Syntax is
:attach <path>, where path is a valid file. Attachments will show up in
the pre-send review screen.