Add HTTPS configuration

[replay]

This PR is based on #693

[breed808]

I'm also very keen to see HTTPS support in the windows_exporter, but the use of two logging libraries will cause confusion for users.
Those running the exporter from the MSI install will have most logs sent to Windows Event Log, while HTTPS-related events sent to STDOUT will be effectively dropped.

It's not a problem with an easy solution, as the exporter-toolkit/https library requires go-kit/kit/log which doesn't currently support Windows Event logging.

[replay]

I'm also very keen to see HTTPS support in the windows_exporter, but the use of two logging libraries will cause confusion for users.
Those running the exporter from the MSI install will have most logs sent to Windows Event Log, while HTTPS-related events sent to STDOUT will be effectively dropped.

That's definitely not optimal

It's not a problem with an easy solution, as the exporter-toolkit/https library requires go-kit/kit/log which doesn't currently support Windows Event logging.

Afaict we have the following options:

1. Use mixed loggers, which is what this PR implements, you have already pointed out the disadvantages of this.
2. Use a different HTTPS server than the one from the exporter-toolkit, that's what I implemented in this PR.
3. Update the exporter-toolkit and make its HTTPS server use the logger from github.com/prometheus/common/log, I don't know enough about that library to judge how difficult/easy that would be and whether such a change would get accepted.

[carlpett]

What I've done on the half-finished work that I mentioned in the other PR is a fourth option, namely a wrapper. I can try to get that finished tonight, since there appears to be much interest in this.

[replay]

Closing this because #693 has been merged