v0.15.0

• Add prog_item_number field containing the raw Program Item Number
• Add partial_alt_kilohertz field containing an incomplete list of
  alternative frequencies when the --show-partial option is used
• Add configure option --without-macports to disable directory checks
  when cross-compiling
• Reduce write calls to output JSON stream using a streamstring (#56)
• Remove an old shell script that launches rtl_fm - instead, there is
  a mention of rtl_fm usage in the readme and wiki