Ignore files in examples (other than .sh and .md)

When downloading files in examples directory (e.g. when running
`prepare-iwslt14.sh`), git sees them as untracked files but they should
not be committed.
Add a .gitignore script that ignore everything in the examples
subdirectories except for .sh and .md files.

examples/.gitignore

+*/*
+!*/*.sh
+!*/*.md