Fallback for unsupported browsers about log

[adamschwartz]

In non-supported browsers, the fallback is you see the extra syntax. So for example, you would see _italic_ instead of italic.

Unfortunately, at the moment, it's not possible to detect CSS support for console.log in any browser. So the approach we've taken is to do some specific browser version testing to make this work. Not great, we know. But since log is meant as a development tool (with some safety, since if you accidentally committed log('_foo_') you wouldn't break any pages), we're OK with this until a feature test is available.