Add @sethvargo as a maintainer

MAINTAINERS.md

@@ -21,5 +21,6 @@ project lead.
 * [Jay Mundrawala](https://github.com/jdmundrawala)
 * [Lamont Granquist](https://github.com/lamont-granquist)
 * [Scott Hain](https://github.com/scotthain)
+* [Seth Vargo](http://github.com/sethvargo)
 * [Steven Danna](https://github.com/stevendanna)
 * [Yvonne Lam](http://github.com/yzl)