Add characteristics command

With that command we can iterate over all characteristics of specific
service.
Implement get_characteristic_callback

When we run characteristics command, this tool will iterate over all
characteristics of specific service. The result of that command will be
handled at get_characteristic_callback.
Add characteristic cache

In order to know which characteristic user wants, we are going to store
in a static buffer and show an ID to each one. This makes user's life
easier than type every characteristic field.