add Insecure flag --insecure to ignore TLS errors.

[gprime31]

trafficstars

tls: failed to verify certificate: x509: cannot validate certificate for https://xxx.xxx

[MMquant]

I have same problem

error: Get "https://some-domain/js/app.5176f0f3.js": tls: failed to verify certificate: x509: certificate is valid for another-domain, *.another-domain, not some-domain

[twest-bf]

Proposed fix in #23