Named Formats Redux

http://haacked.com/archive/2009/01/14/named-formats-redux.aspx/
Don't need the Console Project
Rename StringLib to Interpolate
Rename UnitTests to Interpolate.Tests
Rename NamedStringFormatSolution.sln to String.Interpolate.sln
Switch from xUnit to NUnit
Test each Formatter in its own TestFixture
String.Interpolate is now an extension method
Use GitHub's VisualStudio.gitignore
README.md and LICENSE.md
Enable NuGet package restore
Configure the build environment w/ Homebrew and `direnv`
Run NUnit and NuGet from in a shell
Release Interpolate as String.Interpolate on NuGet
Automate the build, test and release with `make`