Include dist/ files in language stats on GitHub

GitHub considers Bootstrap files to be vendored files, and excludes them from
repository language statistics. This makes sense for most repositories, where
Bootstrap is indeed third-party code. This repository is the exception; it's
first-party code here, so should count toward the language statistics.

.gitattributes

@@ -7,3 +7,7 @@
 *.md    text eol=lf
 *.svg   text eol=lf
 *.yml   text eol=lf
+
+bootstrap-theme.css linguist-vendored=false
+bootstrap.css linguist-vendored=false
+bootstrap.js linguist-vendored=false