Leading whitespace is lost during parsing

Thanks for building `pip-requirements-parser`! I'm really happy with it.

I noticed that if I parse a requirements file and then print it out with `dumps`, any leading whitespace before a requirement or comment gets removed. Example below:
```
(env) tetsuo@Alexs-MacBook-Pro pip-audit % cat requirements.txt
flask==0.5
(env) tetsuo@Alexs-MacBook-Pro pip-audit % python
Python 3.9.13 (main, May 24 2022, 21:13:51)
[Clang 13.1.6 (clang-1316.0.21.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from pip_requirements_parser import RequirementsFile
>>> rf = RequirementsFile.from_file("requirements.txt")
>>> rf.dumps()
'flask==0.5'
```
It seems that whitespace within the line is preserved, it's just when it exists at the beginning of the line that it gets stripped.
```
(env) tetsuo@Alexs-MacBook-Pro pip-audit % cat base.txt
django-celery-results==2.3.1 \
--hash=sha256:b8c9416619dbcc38f13398e31bcb1f14a228cd1e8f65fb22d3b7fc68aaa5331a \
--hash=sha256:bf24ecc29c42e49cc7eb30b9b3739471331e2a0ca517cc88ca53a0cf3a2031d1

(env) tetsuo@Alexs-MacBook-Pro pip-audit % python
Python 3.9.13 (main, May 24 2022, 21:13:51)
[Clang 13.1.6 (clang-1316.0.21.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from pip_requirements_parser import RequirementsFile
>>> rf = RequirementsFile.from_file("base.txt")
>>> rf.dumps()
'django-celery-results==2.3.1 \\\n --hash=sha256:b8c9416619dbcc38f13398e31bcb1f14a228cd1e8f65fb22d3b7fc68aaa5331a \\\n --hash=sha256:bf24ecc29c42e49cc7eb30b9b3739471331e2a0ca517cc88ca53a0cf3a2031d1'
```
This matters to `pip-audit` because our use case involves parsing a requirements file and then rewriting it with some of the requirements modified. So any requirements files that have leading whitespace will have the whitespace removed when we rewrite a requirements file.

The first example that comes to mind are automatic comments that are indented like those inserted by [pip-compile](https://github.com/jazzband/pip-tools).

Are you interested in having `pip-requirements-parser` support this use case? If so, I'm happy to take a stab at a PR.