Support basic auth via header and/or ~./netrc file

[leonsery]

Currently for Prometheus servers that uses basic authentication we can use promi with -u parameter, for example:
promi -u http://user:pass@localhost:9090 targets

It will be great if you will add the ability to use Authorization header and/or ~/.netrc file like in curl, for example:
promi -H "Authorization: Basic dXNlcjpwYXNzCg==" targets
promi -n targets # assuming there is ~./netrc file

[zbindenren]

Good idea, I will implement this when I am back from vacation.