mailbox : add option to prevent platform EOL conversion

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-05-20.12:09:06.362>
labels = ['3.7', 'type-feature', 'library']
title = 'mailbox : add option to prevent platform EOL conversion'
updated_at = <Date 2017-05-21.18:25:15.801>
user = 'https://bugs.python.org/adder'

bugs.python.org fields:

activity = <Date 2017-05-21.18:25:15.801>
actor = 'r.david.murray'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-05-20.12:09:06.362>
creator = 'adder'
dependencies = []
files = []
hgrepos = []
issue_num = 30412
keywords = []
message_count = 2.0
messages = ['294028', '294108']
nosy_count = 2.0
nosy_names = ['r.david.murray', 'adder']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue30412'
versions = ['Python 3.7']