Add Mtime plugin

This plugin adds last modification time of each file to its metadata.

I thought that, since to get proper file mtimes from git one needs some [additional tools](https://git.wiki.kernel.org/index.php/InterfacesFrontendsAndTools#Backups.2C_metadata.2C_and_large_files), this plugin won't be of much use to anyone except me. But because there are already [some downloads](http://npm-stat.com/charts.html?package=metalsmith-mtime), maybe others also find it useful.