#FirefoxKlar 1 hashtag

Two more minor changes in #serci:

1. You may set the default search-engine now via the "default_search" GET argument. This is useful for private browser windows or browser like #FirefoxKlar. Simply set your search engine to https://search.jayvii.de/?default_search=metagerweb&q=%s and the default search engine will be MetaGer. More: https://src.jayvii.de/pub/serci/commit/c11ef8fd6539487afd95c6ff767f3e2bdf663cc5.html
2. I also added the #nona search engine to the default configuration file. More: https://src.jayvii.de/pub/serci/commit/d23d91f19df8bd43132a9363f3cf7501afbc3f0a.html