Ignore dirty vendor directory

Under most circumstances, this pull request eliminates
```
#	modified:   vendor (modified content)
```
in the output of `git status` and
```
-Subproject commit 6afe43df27d38fc561f49ff4bd96a8d2bf5da33f
+Subproject commit 6afe43df27d38fc561f49ff4bd96a8d2bf5da33f-dirty
```
in `git diff`. It still allows you to see those if you want—you just have to cd to the vendor directory first.