Add ability to provide a custom IP checker URL.

[pgreenland]

Hi,

After being hit by the recent opendns issues, I thought I might switch to a private IP source, such as my home router.

Rather than just hack in my new URL I thought it might be useful for others to add the ability for a custom URL to be provided.

This patch adds a new IPCHECKER "custom" with an associated parameter IPCHECKER_CUSTOM_URL, specifying the URL to request the external IP from.

I've added HTTP support too, selected based on the URI scheme, with a warning generated if HTTP is used.

Documentation updated (along with a few typos), container built and published to my dockerhub account (pgreenland/route53-dynamic-dns:latest), have tested with the default and my custom handler.

FYI I'm not normally a node.js / javascript developer, so I may have done something monumentally stupid somewhere. Happy for you to suggest or make any changes required to include it.

Great project btw, saved me a ton of work!

Thanks,

Phil