Add hexadecimal binary output conversion option

This commit adds another binary output conversion option to convert
binary blobs into hexadecimal notation, similar to the output of the
following command:
  xxd -p binaryfile.bin | tr -d '\n'

(In other words, a single line string of hexadecimal characters
representing a binary blob)

When exporting SQL INSERT statements for SQLite and PostgreSQL and when
the hexadecimal notation is specified with 'mdb-export -b hex ...',
special consideration is given to ensure that binary blobs are safely
written to the SQLite or PostgreSQL database.
Fix #37: configure error for ENABLE_SK conditional not being defined

Suggested fix by roytam1 at:
https://github.com/brianb/mdbtools/issues/37#issuecomment-29264591