Adding non-default gdb handling

[bannsec]

Ref #1071 -- this minor change allows users to override the default gdb front-end with something else. I have tested it with a simple python wrapper that instead connects up with radare2. Works pretty slick.

[zachriggle]

Would you mind rebasing this? We had some issues with the tests on dev. Could you also add Fixes Gallopsled/pwntools#1071 to the end of the commit message? Thanks!

Git Tip

In case you're not familiar with git rebase and changing commit messages git commit --amend:

# Check out the branch with your work
$ git checkout non-gdb

# Get a copy of the updated pwntools dev branch, and re-play your changes 
# instead of merging them with '--rebase'
$ git pull --rebase https://github.com/Gallopsled/pwntools dev

# Update your commit message
$ git commit --amend

# Push your updated 'non-gdb' branch to Github, using '--force' to overwrite the old branch
$ git push --force https://github.com/bannsec/pwntools non-gdb

[zachriggle]

Actually, nevermind, I think all of that can be ignored and I can do it trivially from the Github UI.