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/markdown.micro

@@ -1,4 +1,4 @@
-syntax "Markdown" "\.(md|mkd|mkdn|markdown)$"
+syntax "markdown" "\.(md|mkd|mkdn|markdown)$"
 
 # Tables (Github extension)
 color type ".*[ :]\|[ :].*"