Fix output format of optional BED fields

I recently posted an issue in the discussion board regarding mergeBed (https://groups.google.com/forum/#!topic/bedtools-discuss/ksBqfvSW1eA). Basically, depending on the options used, the output format of the optional fields such as the score and strand appeared to be a little inconsistent. This may cause formatting problems if used as input for other downstream bedtools applications. I have proposed changes such that the optional BED fields (namely name, score and strand) are outputted in their designated columns when applicable. Hope they will be useful, thanks!