Make all filetype names lowercase

This may be a breaking change if you are using a plugin that
checks the filetype. All the default plugins are now updated,
just make the filetype you were checking for previously all
lowercase.

runtime/syntax/python.micro

@@ -1,6 +1,6 @@
 ## Here is an example for Python.
 ##
-syntax "Python" "\.py$"
+syntax "python" "\.py$"
 header "^#!.*/(env +)?python( |$)"
 
 ## built-in objects