ftplugin: Only load the python plugin once

This makes opening wiki pages significantly faster and also prevents
clearing the entire cache every time.

tests/test_ftplugin.py

@@ -0,0 +1,18 @@
+import os
+from tests.base import IntegrationTest
+
+
+class TestCachePreserved(IntegrationTest):
+
+    def execute(self):
+        testfile2 = os.path.join(self.dir, "testwiki2.txt")
+
+        self.command("w", regex="written$", lines=1)
+        self.command("w {}".format(testfile2), regex="written$", lines=1)
+
+        self.client.edit(testfile2)
+
+        # check that all buffers have a cache entry
+        cache_keys = self.client.eval('py3eval("sorted(cache.caches.keys())")')
+        buffers = self.client.eval('py3eval("sorted(b.number for b in vim.buffers)")')
+        assert cache_keys == buffers