Fix a couple README typos.
Change feed link from "Continue reading ..." to "More&hellip;"
Launch browser for interactive dev.

(I'd disabled it while working for awhile via SSH to a dev box.

This is N/A for real users. Just the build/preview function I use
sometimes for interactive development.)
Allow source and output directories to be specified. Closes #57.

- Add .frogrc user variables `source-dir` and `output-dir`. Default
  respectively to `_src` and `.` for status quo behavior. Allow each
  to be an absolute path, or, relative to the project top dir.

- Clean up some confusing and misnamed code in `paths.rkt`.

  - Normalize paths for things like `~`, `.`, and `..`.

  - Add more unit tests here, too.

  - Use `abs->rel/src` when you need to convert an absolute local path
    into a path string that is relative to the source dir.

  - Use `abs->rel/www` when you need to convert an absolute local path
    into a path string that is relative to the output (www) dir.

Thanks to @paul-meier for the original pull request. I was nearly able
to use it as-is, but made a fresh start in order to clean up some mess
on my end that made it harder to add this than it should have been.