Add ability to set custom HTML

[jamesfowkes]

With reference to issue #12 , a simple implementation of user-settable custom HTML.

Defaults to using the default WebOTA header, but can be overridden by use of the set_custom_html method.

I kept the changes as simple as possible.

[scottchiefbaker]

This looks good to me. For consistency with the rest of the code can you rework the braces on the if/then on line 155 and the function definition on 226 to be the same brace style as the init_mdns() function.

Once everything is consistent I will gladly land this. Thanks for taking the time.

[scottchiefbaker]

Someone on #github suggested that I send you a PR on your branch to suggest the style fixes. I clicked the wrong button, and it committed to your repo instead of starting a pull-request. Sorry, I'm new at remote branch stuff.

See: dfbaf8e

This is now complete, and looks good. I'll merge it!

[jamesfowkes]

That's cool, I press the wrong buttons/enter the wrong commands all the time! Glad you were happy with it!