fix(index): do not emit sourceMap warning when explicitly set to false

Author: yyx990803

Diff for: lib/index.js

@@ -137,7 +137,7 @@ module.exports = function loader (css, map, meta) {
       css = this.exec(css, this.resource)
     }
 
-    if (!sourceMap && map) {
+    if (!sourceMap && sourceMap !== false && map) {
       this.emitWarning(`\n\n ⚠️  PostCSS Loader\n\nPrevious source map found, but options.sourceMap isn't set.\nIn this case the loader will discard the source map entirely for performance reasons.\nSee https://github.com/postcss/postcss-loader#sourcemap for more information.\n\n`)
     }