a useful static file url-to-graph dot viewer

* People can link to it. For example: http://fiatjaf.alhur.es/viz.js/?url=https://fiatjaf-annex.s3.amazonaws.com/SHA256E-s1895--4d621b28b8477887eb3c4d2974ad868ffeb9ff4c31d334882d8095fa8bb78377.dot
* Invalid input gives a nice error: http://fiatjaf.alhur.es/viz.js/?url=http://google.com/search
* There are options for engines and layouts, just send `format` or `engine` or `title` in the query string.
* Works with a normal query string and a query string sent after the hash, as in `#url=http://example.com/graph.dot`