deprecate string option instead of removing it

Author: sokra

lib/SourceMapDevToolPlugin.js

@@ -20,6 +20,12 @@ class SourceMapDevToolPlugin {
 	constructor(options) {
 		if(arguments.length > 1)
 			throw new Error("SourceMapDevToolPlugin only takes one argument (pass an options object)");
+		// TODO: remove in webpack 3
+		if(typeof options === "string") {		
+			options = {		
+				sourceMapFilename: options		
+			};		
+		}
 		if(!options) options = {};
 		this.sourceMapFilename = options.filename;
 		this.sourceMappingURLComment = options.append === false ? false : options.append || "\n//# sourceMappingURL=[url]";