Refactor key extraction into configuration file

[macdonst]

Different push services end up naming their keys differently or putting the keys in different sub objects. We should refactor the key extraction code so that it loads the key definitions from a configuration file so that we can support more backend push services easier.

[macdonst]

@smdvdsn let's use the issue230 branch to collaborate on it. I wanna finish out release 1.4.0 first but then I will get on this.

[smdvdsn]

ok thanks @macdonst One thing I wanted to ask is do you want to support everything out of the box? So as new providers are discovered are you thinking that the default config on master would be updated to include the new keys or do you want to only support the basic keys and then allow people to configure from there?

The main reason I ask is about backwards compatibility. Basically merging #220 as is might lock us into supporting these providers in future versions. Then again I could just be over thinking it.

[macdonst]

Let's put everything into the configuration file for now. I like to make it as easy as possible for people to pick up this plugin and get going. If it becomes too crazy to support then we drop it during a major version as we can call it a breaking change at that point and just document the crap out of it.