add support for .tif format; changed regex to use global file_format var for src/dst image loading

needed to reinstall imagemagick w/ tiff support, via Homebrew:
brew install imagemagick --with-libtiff

great help here:
http://stackoverflow.com/questions/12943947/imagemagick-can-not-find-del
etegates-library-for-tiff-format-on-mac-os-x-mounta