Allow to override exclude in config

[MasouShizuka]

I hope to use indent in markdown, because I am used to using list item to record knowledge in a hierarchical manner.

Of course, I can modify the source code directly, but I hope to be able to do it in a configuration way. For example, use vim.tbl_deep_extend rather than vim.list_extend.

[glepnir]

markdown should remvoe from default ..its a common filetype