Add `find_links` support (enhanced)

[devmonkey22]

This PR is designed to supersede PR #23 with several fixes:

• Fix find_links to not drop URL parseable links and only process list once.
• Add unit test for new --find-links argument.
  • Validate that we don't lose find_links when multiple packages are contained in requirements file.
• Update --index-url to default using PIP_INDEX_URL environment variable like pip can, otherwise default to pypi or Asia index (like original code) depending on timezone. If timezone unavailable (errors), fallback to pypi too.
• Fix tox config to include beautifulsoup4.
• Update to version 0.4.3.dev1 as a placeholder.