Switch string substitution to the strings package. Give extensible substitution and dates.

Ok, this you may or may not like this pull request, but I think it makes sense. The [strings package](https://github.com/sellside/strings) utilizes an extensible middleware pattern to extend string substitution. It also has moment.js built it to handle dates. By switching to this package, and providing date usage based on the "date" metadata, we now have the ability to do things like:
```
===
date: 2014-10-09
title: Hello World
===

hello
```
and...
```
"metalsmith-permalinks": {
    "pattern": ":YYYY/:MM/:DD/:title"
}
```
to get...
```
build/2014/10/09/hello-world
```

An addtional interface could be opened for plugins to define additional permalink tokens using the strings package's pattern matching.

I also fixed newlines on the tests.