Add support for text (i.e. non-binary) keys files.

Currently, the only way to generate a perfect hash table is by providing a keys file, which is expected to be a binary file that is an array of, for example, ULONGs.  This is efficient, but not that easy to work with if you want to view or otherwise interact with.

It would be nice to have the option to pass a text file that can be used for keys instead.