Option --trusted-host is not copied from requirements.in to requirements.txt

[atugushev]

Environment Versions

1. OS Type: Mac OS
2. Python version: Python 3.7.3
3. pip version: pip 19.3.1 from /.venv/lib/python3.7/site-packages/pip (python 3.7)
4. pip-tools version: pip-compile, version 4.2.0

Steps to replicate

1. echo "--trusted-host exmaple.com" | pip-compile - -qo-

Expected result

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file=- -
#
--trusted-host exmaple.com

Actual result

#
# This file is autogenerated by pip-compile
# To update, run:
#
#    pip-compile --output-file=- -
#

Additional context

Caught by @altendky, see #824 (comment).
Related issues/PRs: #338, #371