Add support for `--no-strip-markers` in `pip compile` output (#4503)
## Summary
This is an intermediary change in enabling universal resolution for
`requirements.txt` files. To start, we need to be able to preserve
markers in the `requirements.txt` output _and_ propagate those markers,
such that if you have a dependency that's only included with a given
marker, the transitive dependencies respect that marker too.
Closes #1429.
