Add interview questions

Yay questions!

Can you check that I didn't duplicate any of the questions? A set of fresh eyes would be great since there's so many questions now. I put some questions under two sections (e.g. Linked Lists + Trees/Graphs).

Also, I didn't write out any function headers. I've never had an interviewer give me a specific function header to adhere to (and questions can be slightly modified based on follow up questions). I think people who look through the README will get the gist of what's being asked.

I alphabetized the types of questions and added a grab bag section and non-coding section at the end. The grab bag questions *can* be open ended as to how they're implemented, so that's something I want people to think about as they're solving it rather than knowing it was organized under "arrays" or "trees". :dog: 

My sources were a mix of friends, recruiters, interview question websites, and glassdoor.