Use filenames as a definitive answer (#2006)

* Separate find_by_extension and find_by_filename
find_by_extension now takes a path as argument and not only the file extension.
Currently only find_by_extension is used as a strategy.

* Add find_by_filename as first strategy

lib/linguist.rb

@@ -59,8 +59,9 @@ class << Linguist
   # Strategies are called in turn until a single Language is returned.
   STRATEGIES = [
     Linguist::Strategy::Modeline,
-    Linguist::Shebang,
     Linguist::Strategy::Filename,
+    Linguist::Shebang,
+    Linguist::Strategy::Extension,
     Linguist::Heuristics,
     Linguist::Classifier
   ]