Support for http/socks proxy

Currently we do not exclusively support http proxy because of using svn to fetch git tags. Though, it is possible to configure the `svn` binary to use proxied settings, it is not the best approach.

Possible fixes:
- Ship with a local svn binary and allow explicit ``http_proxy`` in the config file (``.env``) and use it project wide
- Somehow use both svn and github api to fetch the tags in suitable situations
- Implement basic svn client protocol in python
- ??

Related:
- https://github.com/nexB/vulnerablecode/issues/554
- https://github.com/nexB/vulnerablecode/issues/556
- https://github.com/nexB/vulnerablecode/issues/568