Hardcoded references to api.thingspeak.com and thingspeak.com

[davidjb]

The current code base has a few locations where api.thingspeak.com or thingspeak.com are hard-coded for either loading data in JavaScript, charting or in the documentation. In some locations, either constants or template variables are already used in similar areas, so this would just be an extension of ensuring that they're used everywhere. Overall, this would make the code and documentation more flexible and portable for self-installs.

Would a PR be accepted that cleans these up and uses template variables and/or contstants based upon environment variables? The default can always match the current implementation (eg default to thingspeak.com etc).

From #37, the current version of thingspeak.com differs from what's currently on GitHub so it would be good to know whether there are updates coming from that version any time soon -- avoiding reimplementation would be good.