use and expose largeSource flag in convert-source-map

it is necessary for huge files. Related issues
v8 https://bugs.chromium.org/p/v8/issues/detail?id=3878
nodejs nodejs/node#759
convert-source-map https://travis-ci.org/thlorenz/convert-source-map/jobs/53989952
gulp-sourcemaps gulp-sourcemaps#73

index.js

@@ -33,7 +33,7 @@ module.exports.init = function init(options) {
       var sourcePath = ''; //root path for the sources in the map
 
       // Try to read inline source map
-      sourceMap = convert.fromSource(fileContent);
+      sourceMap = convert.fromSource(fileContent, options.largeSource);
       if (sourceMap) {
         sourceMap = sourceMap.toObject();
         // sources in map are relative to the source file