Allow specifying tasks to run before wct:{local,sauce}
Allow specifying tasks to run before wct:{local,sauce}

If some files need to be built before web-component-tester runs, this lets users specify the tasks that build them.

hvdb/web-component-tester,hvdb/web-component-tester,marcinkubicki/web-component-tester,fearphage/web-component-tester,fearphage/web-component-tester,Polymer/tools,hypnoce/web-component-tester,Polymer/web-component-tester,Polymer/tools,hypnoce/web-component-tester,hvdb/web-component-tester,marcinkubicki/web-component-tester,sankethkatta/web-component-tester,Polymer/tools,marcinkubicki/web-component-tester,sankethkatta/web-component-tester,Polymer/tools,Polymer/web-component-tester,fearphage/web-component-tester,sankethkatta/web-component-tester,Polymer/web-component-tester