Parameter add.symbols in autoplot() methods

[aphalo]

For historical reasons autoplot() methods do not use the scales defined in the package but have code that assembles axis labels from scratch, and are thus less flexible. This part of the implementation of autoplot() methods needs to be rewritten, and if this is done, then support for an add.symbols would be straightforward. The change to use the scales, most likely will change the plots created without breaking any code. Is not that easy a task. Among other things the scales do not support Markdown but the autoplot() methods do. It is quite likely that nobody uses the Markdown featured as it is rather bare-bones.