New script get_patch_from_maildir to extract patch from maildir.

Signed-off-by: Gris Ge <cnfourt@gmail.com>
get_patch_from_maildir: Allow search by message id

 * Two way to use this script:
    get_patch_from_maildir \
        --maildir ~/.mail/RH/13_LSM/ \
        --msg-id '<5436F310.3050203@redhat.com>'
    get_patch_from_maildir \
        --maildir ~/.mail/RH/13_LSM/
        --subject '[PATCH 0/7] SMI-S plugin refactor (squashed commits)'

Signed-off-by: Gris Ge <cnfourt@gmail.com>