Preserve HISTORY and COMMENT headers

[confluence]

We skipped these to simplify the original implementation, but they are sometimes used to store important metadata, and should not be ignored (see e.g. #1275).

Related to #39 (which would solve the same problem). I think that both options should be implemented and controlled by commandline flags, so that if the original FITS file has very long HISTORY or COMMENT headers they don't have to be duplicated.

[confluence]

Upon reflection, it may be a better idea (and simpler) not to add HISTORY and COMMENT headers separately, and only to make them available via the original FITS string header.