Move configuration to parent project to better support using library as a submodule

[tstellanova]

The problem:

It's often desirable to include BeaconScanner as a submodule within another project. Currently, users can't easily do this because configuring the library to strip some beacon types at compile time requires modifying a file (config.h) internal to the library.

Suggested solution:

Externalize the config file so that the parent (application) project needs to declare which beacon types it's interested in. This allows the parent project to include the BeaconScanner library, unmodified, as a submodule. Update the readme and examples to demonstrate how to include the config file before BeaconScanner.h