Add `edit` command for more security.

Since some third-party GUIs don't play well with the filters, some users have them disabled, and rely on `lock`/`unlock` to work with encrypted files. This, however, is error-prone, since it's likely that one will forget to re-encrypt the file again after they're done editing.

I propose a `git-crypt edit ` command that would decrypt a temporary copy of the file to /tmp/, launch `$EDITOR` to edit it, and then re-encrypt it back to its original location, with a single command (similar to how ansible-vault does it). This would ensure that users would not forget to re-encrypt their files.