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/reST.micro

@@ -1,5 +1,5 @@
 ## For reST
-syntax "RST" "\.rest$" "\.rst$"
+syntax "rst" "\.rest$" "\.rst$"
 # italics
 #color magenta "\*[^*]\*"
 # bold