Implement support for alias templates

This change adds a canonical alias.html template that is used for page
redirects, and passes the page as data to the template under .Page

Fixes #2533
Closes #2576

content/extras/aliases.md

@@ -94,3 +94,11 @@ Assuming a baseurl of `mysite.tld`, the contents of the html file will look some
 ```
 
 The `http-equiv="refresh"` line is what performs the redirect, in 0 seconds in this case.
+
+## Customizing
+
+You may customize this alias page by creating an alias.html template in the
+layouts folder of your site.  In this case, the data passed to the template is
+
+* Permalink - the link to the page being aliased
+* Page - the Page data for the page being aliased

content/templates/overview.md

@@ -71,4 +71,6 @@ Used to render the XML sitemap
 ### [404](/templates/404/)
 This template will create a 404.html page used when hosting on GitHub Pages
 
+### [Alias](/extras/aliases/#customizing)
+This template will override the default page used to create aliases of pages.