Preserve file comment headers

Comment headers in files often contain important metadata about the file
(copyright, license, contact information) that should not be removed.

A note about the implementation: read-comment-header can be implemented
in ~3 lines by using .readLine. However, .readLine discards newlines, so
slamhound would have to make an assumption about what newlines have been
discarded, which is a mistake.

e.g. Editing Unix source files on a Windows machine would produce \r\n
     line terminators if we join the header lines with
     (System/getProperty "line.separator")

PS. I didn't see an appropriate namespace for the new functions, so I
just placed them in slam.hound.