Only call .toString() on sass files

Calling .toString() is a fairly expensive operation, which allocates
a new string, and copies all of the contents of the file Buffer into
that string. This is fairly fast for small files or small sites, but
quickly starts to take seconds of the total build time on medium
sized sites (in my case, roughly 2 seconds of the overall build was
this call to toString()!)

This change makes it so we only call toString() if we actually want
the result - i.e. if it as a sass file. Thus, we don't call it on
markdown files, image files, video files, etc.

lib/index.js

@@ -36,6 +36,17 @@
       },
 
       compile = function(config, basePath, files, filename, done) {
+        if (isPartial(filename)) {
+          delete files[filename];
+          done();
+          return;
+        }
+
+        if (!isAnySassFile(filename)) {
+          done();
+          return;
+        }
+
         var file = files[filename],
             filePath = path.join(basePath, filename),
             opts = extend({
@@ -57,54 +68,47 @@
           opts.outFile = filePath.replace(/\.s[ca]ss/, '.css');
         }
 
-        if (isAnySassFile(filename) === true) {
-          // Append the file's base path to the available include paths.
-          opts.includePaths.push(fileDir);
-
-          // Compile the file using SASS.
-          sass.render(opts, function (err, result) {
-            var error = null;
-
-            if (err && err instanceof Error) {
-              error = new Error([
-                '[metalsmith-sass] Error: ',
-                err.message, ' -> ',
-                err.file,
-                ':',
-                err.line, ':', err.column
-              ].join(''));
-            } else if (err) {
-              error = new Error(err);
-            }
+        // Append the file's base path to the available include paths.
+        opts.includePaths.push(fileDir);
+
+        // Compile the file using SASS.
+        sass.render(opts, function (err, result) {
+          var error = null;
+
+          if (err && err instanceof Error) {
+            error = new Error([
+              '[metalsmith-sass] Error: ',
+              err.message, ' -> ',
+              err.file,
+              ':',
+              err.line, ':', err.column
+            ].join(''));
+          } else if (err) {
+            error = new Error(err);
+          }
 
-            if (error) {
-              done(error);
-              return;
-            }
+          if (error) {
+            done(error);
+            return;
+          }
 
-            // add soure map
-            if (result.map) {
-              files[dest+'.map'] = {
-                contents: result.map,
-                mode: file.mode
-              };
-            }
+          // add soure map
+          if (result.map) {
+            files[dest+'.map'] = {
+              contents: result.map,
+              mode: file.mode
+            };
+          }
 
-            // replace contents
-            file.contents = result.css;
+          // replace contents
+          file.contents = result.css;
 
-            // rename file extension
-            files[dest] = file;
+          // rename file extension
+          files[dest] = file;
 
-            delete files[filename];
-            done();
-          });
-        } else if (isPartial(filename) === true) {
           delete files[filename];
           done();
-        } else {
-          done();
-        }
+        });
       },
 
       compileSass = function compileSass(config, files, metalsmith, done) {