Change require in example.
Redesign to squash many bugs. Version 0.1.0.

- Bugs fixed:
  - Closes #4.
  - Closes #5.
  - Closes #6.
  - Closes #7.
  - Closes #8.
  - Closes #9.
  - Closes #10.
  - Closes #11.
  - Closes #12.

- Add a version to info.rkt so that e.g. Frog can require this
  version. (Although this version probably is 100% backward
  compatible, I'm bumping the minor version so that `raco pkg update
  frog` can know to automatically get this, after I add it as a dep in
  Frog's info.rkt.)

- Add `replace/match`, which is like our plain `replace` except it can
  match things that are already parsed into some mix of x-exprs. Use
  this for things like bold and italic which are supposed to nest
  inside each other, as well as for links which are supposed to allow
  arbitrary markdown in their visible text.

- `read-markdown` is now an alias for `read-markdown/racket`, which is
  our much-fixed-up parser written entirely in Racket. However there's
  also a new `read-markdown/perl`, which shells to markdown.pl to do
  most of the parsing, then adds a couple features such as footnotes.
  There's also an option to run unit tests comparing our Racket output
  to that of markdown.pl; however it's off by default because a dozen
  tests fail for mostly gratuitous differences.

- Add many more units tests; now totaling close to 100.