Add format string and callback to pll_newick_export

[xflouris]

Add two additional parameters to the pll_newick_export function:

1. A format string denoting how nodes will be printed
2. A callback function that extract the necessary node information from the node->data field and passes them, along with the format string, to asprintf.

Also write some standard callbacks, e.g. for printing only branch lengths.

[xflouris]

finished.

pll_{r,u}tree_export_newick now accepts a callback for printing arbitrary data elements. If NULL is passed instead of a callback, the behaviour remains as before, i.e. only branch lengths are printed. newick-export is an example demonstrating the new functionality.