add raw-data/external-data/ to .gitignore file.

This directory is for storing things like raw HTML, which has been downloaded to
reduce making repeated calls to the server to grab the same data. The data in
the directory will not be included in the commit history because I don't want to
clog up the commit history with it. Ideally, I would prefer the code to provide
the functionality to grab the data (on a freshly cloned version of the
repository) if the user needs the data. Otherwise, the cleaned (not necessarily
in its final state) data is what the user will most likely be working with.

.gitignore

@@ -1,3 +1,4 @@
+raw-data/external-data/
 # ---> CommonLisp
 *.FASL
 *.fasl