Don't duplicate site title in home page TITLE tag (#78)

Let's say your `.Site.Title` is "My Website". The code grabs `.Site.Title`, and then then checks to see if there's a `.Title`. When you're on the homepage, `.Title` is equivalent to `.Site.Title`, so that the browser bar will read:

> My Website | My Website

If we qualify `.Title` by confining to the `.Params` namespace, then the site title isn't duplicated:

```<title>{{ block "title" . }}{{ .Site.Title }} {{ with .Params.Title }} | {{ . }}{{ end }}{{ end }}</title>```

> My Website

But it still picks up `.Title` from posts and pages, where a content title exists in front matter:

> My Website | About Me