GDB library + test + interactive support + plugin

Looks like we can review everything at once.

This PR includes what was in the first one,  the Avocado GDB library and its accompanying test, plus the interactive support (user level). 

Oh it would be great if we just use the Namespace `avocado.cli.parser.Parser.args` that we grab from the parsed arguments of the command line, so in this case, the things that is now on the runtime feature, will be available as attribute of args. Of course, this requires a bunch of changes in the code, to pass the arguments namespace to the place where it needs.