Fixes #3 - allow passing paths through stdin

[nat45928]

Changeset works as expected.

(.venv) C:\repos\req-compile>req-candidates --paths-only | req-compile --extra test
astroid==2.3.2             # pylint (<2.4,>=2.3.0)
atomicwrites==1.3.0        # pytest (>=1.0)
attrs==19.3.0              # pytest (>=17.4.0)
certifi==2019.9.11         # requests (>=2017.4.17)
chardet==3.0.4             # requests (<3.1.0,>=3.0.2)
colorama==0.4.1            # pylint, pytest
idna==2.8                  # requests (<2.9,>=2.5)
isort==4.3.21              # pylint (<5,>=4.2.5)
lazy-object-proxy==1.4.3   # astroid (==1.4.*)
mccabe==0.6.1              # pylint (<0.7,>=0.6)
mock==3.0.5                # req-compile[test]
more-itertools==7.2.0      # pytest (>=4.0.0)
packaging==19.2            # pytest
pluggy==0.13.0             # pytest (<1.0,>=0.12)
py==1.8.0                  # pytest (>=1.5.0)
pylint==2.4.3              # req-compile[test]
pyparsing==2.4.3           # packaging (>=2.0.2)
pytest==5.2.2              # pytest-mock (>=2.7), req-compile[test]
pytest-mock==1.11.2        # req-compile[test]
req-compile[test]==0.9.14  # - (==0.9.14)
requests==2.22.0           # req-compile, responses (>=2.0)
responses==0.10.6          # req-compile[test]
six==1.12.0                # astroid (==1.12), mock, packaging, req-compile, responses
urllib3==1.25.6            # requests (!=1.25.0,!=1.25.1,<1.26,>=1.21.1)
wcwidth==0.1.7             # pytest
wrapt==1.11.2              # astroid (==1.11.*)