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/html.micro

@@ -1,6 +1,6 @@
 ## Here is a short improved example for HTML.
 ##
-syntax "HTML" "\.htm[l]?$"
+syntax "html" "\.htm[l]?$"
 color identifier start="<" end=">"
 color special "&[^;[[:space:]]]*;"
 color constant ""[^"]*"|qq\|.*\|"