Add a proxy function

[yuankeyang]

Proxy function can easily fetch the data.

[ikeydoherty]

Sorry you're gonna need to clarify here.. I don't know what you mean.

[yuankeyang]

Because of the internet proxy, I have trouble in fetching the NVD data. So I think it would be more convenient and powerful to have a internet proxy function.

[ikeydoherty]

Oh right. So cve-check-tool just respects the standard http_proxy and https_proxy environmental variables because it uses libcurl :)