Meta: Add gitattributes file

Github Linguist will read this file when calculating language stats for
the repository. We can use this to exclude any vendored dependencies in
externals and dist. Also makes all h files be considered cpp

.gitattributes

@@ -0,0 +1,4 @@
+dist/languages/* linguist-vendored
+dist/qt_themes/* linguist-vendored
+externals/* linguist-vendored
+*.h linguist-language=cpp