Check if override built-in command

[ZetaTwo]

This PR aims to start a discussion on #230
This code fetches current gdb command on start-up and stores them in a list in the Command class.

The question is what to do with this list. As an example, I simply print out a warning but this is not really desirable since it prints out warnings for intended behaviour and is not actionable for the end-user. Another way to go would be to have a whitelist of the currently overridden commands: up, down, search, pwd and start and abort/exit if you try to override any other.

Thoughts?

[disconnect3d]

Hey, thanks for the PR!

I haven't yet looked into the code (will probably do it tomorrow) but I am up for a whitelist of commands that we accept to override. If something doesn't fill into the whitelist, I think we could just fail (i.e. raise an exception or even exit gdb?).

[ZetaTwo]

Great, I updated the PR with the described behaviour. Any attempts to override anything other than the currently overridden commands: up, down, search, pwd and start will raise an exception. This is done with a hard-coded whitelist in the command class. If you prefer a more sophisticated way of doing it, please let me know.

[ZetaTwo]

The TravisCI check fails due to unrelated whitespace issues fixed separately in #544

[ZetaTwo]

Uhm, I messed up with my git:ing, please hold.

[ZetaTwo]

Ok, this should address all the points discussed. The solution for saving the pagination value is not beautiful but I didn't find a better way.

[disconnect3d]

Thanks!

[ZetaTwo]

Thanks for the merge and for a great project!