New example sketch for custom headers

[jimwhitelaw]

This example shows how to perform several opearations for using custom (non-OBD2 standard)
queries.

Custom Header
We perform a query that requires a custom header to be sent first. The custom header in this
case determines which ECU in the vehicle's system we want to query.

Custom PID
The query sent to the vehicle is also a custom (non OBD2 standard) PID using the 0x22
(enhanced data) mode.

Manually Processing Response
In this example, we manually extract the data value from the query response and perform
some post-processing to calculate the correct value.

Managing Query State
We also demonstrate managing the query state used by the loop() method. This is typically
managed internally by ELMduino for standard PID methods.

Customize all the things!
The header value, PID, data value bytes and adjustment formula are generally unique for
each different vehicle and PID. You will need to source the correct values for those for
your specific vehicle. This example almost certainly will not work for you "as-is".