Add support for TIFF input

Thanks for this great tool! Would it be possible to add support for using TIFFs as the input? I'm a web developer at an art museum, working on automating image conversion for our website. It's pretty much the universal standard in the museum world to use TIFF as the archival format for object images. I could convert TIFFs to PPM and take it from there, but I'm running into some color balancing issues (which I'll fix shortly), and I feel like other folks out there might also appreciate the option to use TIFFs as a default, so that there's no need to bring in other utilities into the tool chain.

Cheers, and thanks again!

**Edit:** For the record, I fixed my issue with ImageMagick by extracting the color profile(s) from the TIFF before conversion to PPM, converting PPM to JPEG using jpeg-recompress, then applying the saved profile(s) to the JPEG. Here's the sequence of commands:

```
convert in.tif profile.icc
convert in.tif temp.ppm
jpeg-recompress --ppm temp.ppm out.jpg
convert out.jpg -profile profile.icc out-with-profile.jpg
```

I'll work on making this more efficient, but this is a valid work around for the moment.

The issue names no files, tests, or entry points. Start by locating the existing input-format handling and checking how TIFF decoding and color profiles would fit the current C toolchain. Done means TIFF input works without the ImageMagick/PPM workaround and produces the expected converted output.

Written by the indexing model from the issue text.