isPermaLink configuration

[XhmikosR]

Per the specs

If the guid element has an attribute named "isPermaLink" with a value of true, the reader may assume that it is a permalink to the item, that is, a url that can be opened in a Web browser, that points to the full item described by the element.

Personally, I always have that true since it fits my use cases. But, the default currently is false.

So, we could either change the default to true or allow the user to configure this.

[benbalter]

The logic makes sense, but I believe it'll have to be determined by if we have the base URL either via site.url or site.github.url (otherwise we'd only have a relative path).

[pathawks]

If we go the Atom route then this is irrelevant.

[pathawks]

Since #24, I don't think this applies any longer; Atom treats id and link as two separate elements.

[pathawks]

I really believe this can be closed, now that the project has switched to Atom.