libudis86: introduce asmvprintf hook

Introduce asmvprintf hook to implement custom assembly code printing
procedure with the help of udis86 builtin syntax formatting machanism.

For syn-intel/att.c code, a syntax element type (enum ud_syn_class)
is passed as the second argument on each calls to asmprintf().

An example of colorized assembly code output is implemented in udcli.c

Signed-off-by: vardyh <vardyh.dev@gmail.com>