Fix issues on Windows

.gitattributes

@@ -0,0 +1 @@
+test/fixtures/**   text eol=lf

lib/index.js

@@ -3,6 +3,7 @@ var basename = require('path').basename;
 var debug = require('debug')('metalsmith-markdown');
 var dirname = require('path').dirname;
 var extname = require('path').extname;
+var join = require('path').join;
 var markdownIt = require('markdown-it');
 
 /**
@@ -30,7 +31,7 @@ function plugin(preset, options){
       var data = files[file];
       var dir = dirname(file);
       var html = basename(file, extname(file)) + '.html';
-      if ('.' != dir) html = dir + '/' + html;
+      if ('.' != dir) html = join(dir, html);
 
       debug('converting file: %s', file);
       var env = {};