Add package debugging mode

[pazdera]

In many cases it would be useful to be able to manually intervene to try different things during a package build in case something is wrong without having to go through the whole commit-push-build cycle.

To overcome this we could add a debugging mode, where dr would install all the dependencies, and yield you shell access to the build chroot with the code prepared for editing. After making the changes, it would allow you to build, push and repeat the process as many times as necessary.

The debug console could look like this:

dr debug kano-init
... installing dependencies ...
... preparing build root ...
... done ...

Debugging console for kano-init.
Type help for the list of supported commands, or exit to exit.
dr debug> help 
  help        display this
  exit        exit the console
  build       test-build the package
  push        push the test-build to scratch
  edit        change the package sources
  diff        show the changes made
dr debug> edit
root@root:/kano-init$ nano debian/control
root@root:/kano-init$ nano kano-init
root@root:/kano-init$ exit
dr debug> build
... building kano-init ...
dr debug> push
... pushing kano-init to scratch ...
dr debug> diff
+abc
-cba
dr debug> exit

cc @alex5imon @zsero