Replace ~strs with either &'static strs or .to_owned() strs where appropriate
Fields now seem to be private by default - good!

Also, reverse the way columns are rendered: before, a column took a stat and a name to render; now, a file takes a column type to render. This means that most of the File data/methods can be private.

Also add command-line-option-handling code.

Thanks to #rust for helping me diagnose lifetime problems!