It has always been possible to pass a string to the converter in the URI, but that was never documented.
Now it is, and you can pass a string using the q parameter. For example, http://rishida.net/tools/conversion/?q=Crêpes. You can also pass a string with escapes in it, but you will need to be especially careful to percent escape characters such as &, + and # which affect the URI syntax. For example, http://rishida.net/tools/conversion/?q=CrU%2B00EApes.