Add the `a0` pin

[jessebraham]

This pin currently isn't available in the HAL; whenever it is, create the appropriate pin mapping in src/pins.rs.

[brookman]

Hello @jessebraham
I am new to microcontrollers and just checked out this crate to get development going with rust on the d1 mini. The first thing I ran into was the missing a0 pin. Would it by now be feasible to implement it in this crate? I don't understand the whole eco system very well but I have seen that there is a pub struct Analog; in the esp8266-hal-0.4.0 crate (in gpio.rs). Does this already suffice or does there have to be more support in the hal crate?