Add a proxy option for api calls

[tigre-bleu]

I had a use case where my project needs to use a proxy for API calls outside the internal network.

To cover this, I added an optional option to pass the proxies object across nvdlib calls down to python requests.

Example:

proxies = {'http': 'http://proxy.local:3128', 'https': 'http://proxy.local:3128'}
cves_list = nvdlib.searchCVE(cpeName = my_cpe , proxies=proxies)

[cddmp]

@tigre-bleu you can pass a proxy also without that. Just pass the HTTPS_PROXY environment variable.