More robust info

give the ability to view one particular property. show all attributes in the manifest. display in html by default or json if accept header is set
I tried this out in one of my apps, and it made it clear that I need to clean up that app's manifest. In the process of the shadow JAR being built, all kinds of junk ended up in my JAR's manifest. I think it's OK that most of them were displayed under /buildinfo, but what do you think about blacklisting `Manifest-Version` and `Main-Class`?