add a 'deprecated' class to fields which have been marked as such

[TimLehner]

This allows more flexible styling of deprecated fields, e.g. using the following CSS to strikethrough the deprecated fields:

.graphiql-explorer-node.deprecated {
  span {
    span {
      text-decoration: line-through;
      font-style: italic;
    }
  }
}

[sgrove]

This is great, thank you!

[sgrove]

lgtm

[sgrove]

Note that I changed the classname in #41 to be more specific and slightly changed where it was applied, graphiql-explorer-deprecated. This should make it less likely to collide with existing CSS and also make targeting just the field nodes easier.