expose fingerprint as an optional parameter

[proppy]

It'd be nice to have the fingerprint as an optional .begin() parameter to prevent issue like #238 #237 #236

[tomeryehu]

You right!

Also a way to know when its change and how to take the new one ....

[path9263]

I've done a bit of looking around trying to solve this one. It seems it is an issue with the ESP8266HTTPClient, see here. It looks like it should be possible to simply not have the ESP8266 not check if the cert is valid but that is obviously not nearly as secure and hasn't been implemented, you would have to hack it in.