add HTTPS option to omelasticsearch

The option to send data over HTTPS is good when you have Elasticsearch behind an Apache or something else that secures the channel.

I've attached a commit that should add a new "usehttps" boolean option that allows just that. Basically, it controls whether the address begins with "http" or "https" and libcurl seems to do the rest. It worked for me, both for HTTPS and plain HTTP.

That's the good news. The bad news is that it's only my 3rd piece of C code. Ever. After hello world and some other 10-line "program". So please have a look and see if any bad practice slipped in my copy-pasting.

Thanks!

P.S. If this is OK, I'll add another pull request with the doc changes. I believe that's the right thing to do, no? Since rsyslog-doc is a separate project.