Add option to allow support for self-signed SSL certificates

Adds a `crawler.ignoreInvalidSSL` options which sets the following options on the https request:

- `client.rejectUnauthorized = false;`
- `client.strictSSL = false;`

Allows, in conjunction with the `NODE_TLS_REJECT_UNAUTHORIZED` environment variable, the use of self-signed SSL certificates. I needed this to run against a https://localhost site which had a self-signed cert.