Make keyword completions context sensitive

Split out of #34 where @mattgodbolt proposed:
> Moving forward suggesting PRINT halfway through a line should be avoided too. I think we get some clues for those in the "flags" of the tokenisation table (which we parse but don't use yet). e.g. http://www.benryves.com/bin/bbcbasic/manual/Appendix_Tokeniser.htm stuff in there we aren't yet using.