SRD: support setting per-PD options

All decoder class options are automatically copied to a newly created
instance with their default value, which can be overridden with the
GHashTable passed to srd_instance_new().

Currently, only strings and integers are supported as option value
types. The type is set by the default value in the decoder class, and
enforced when overriding them. Integers can be specified in any
format python allows: 10, 0x0a and so on.