Implement geom_auto

[hadley]

Should be S3 generic that dispatches on the type of the data argument, which will need to come first.

Helpful for forecast (@robjhyndman) and sf objects.

[hadley]

Possibly should be auto_geom() or auto_layer() since arguments will be rather different to most geoms.

[robjhyndman]

We are using autolayer() in the forecast package (github version). Could change to auto_layer() if you prefer, but we need to decide soon to avoid broken code down the track. I plan to push a new CRAN version of the forecast package in about a month.

[mitchelloharawild]

Note that I used autolayer() for consistency with autoplot().
Happy for it to be auto_layer(), but if this is used, I'd suggest a similar change for auto_plot().

[hadley]

You're right that it should be autolayer() for consistency. Want to do a PR?