Proper Lexer for SDB

[sdimitro]

Using shlex has gotten us pretty far but we've hit enough problems with it that I believe it's time to do proper lexing and parsing ourselves. I already have some code for a lexer that needs to be polished and have more unit tests added. Integrating just that without the parser code should get us pretty far. Once that is in, we can focus on the actual grammar and see if we want to support things like variables or subshells properly - something that would require a lot more design.