Add Markdown and Elixir extensions

[georgeguimaraes]

Related to #63,

I had to extend extensions to use lua require('otter').activate({'markdown'}). The lack of a markdown extension made the command blow up.

And, since I also use Elixir inside Markdown, I added "ex" to the mix.

wdyt?

[jmbuhr]

Perfect, thanks!
Btw. I still think it's strange to have this mapping, do you happen to know if this already exits somewhere? Going the other way around (extension to filetype) is already a thing, but I never found this mapping.

[georgeguimaraes]

Without the markdown mapping, I get this whenever I try to activate({'markdown'}):

Error detected while processing BufEnter Autocommands for "*.ex":
Error executing lua callback: ...e/.local/share/nvim/lazy/otter.nvim/lua/otter/keeper.lua:223: attempt to concatenate a nil value
stack traceback:
	...e/.local/share/nvim/lazy/otter.nvim/lua/otter/keeper.lua:223: in function 'activate'
	/Users/george/.config/nvim/lua/config/autocmds.lua:11: in function </Users/george/.config/nvim/lua/config/autocmds.lua:10>
Error detected while processing BufWinEnter Autocommands for "*.ex":
Error executing lua callback: ...e/.local/share/nvim/lazy/otter.nvim/lua/otter/keeper.lua:223: attempt to concatenate a nil value
stack traceback:
	...e/.local/share/nvim/lazy/otter.nvim/lua/otter/keeper.lua:223: in function 'activate'
	/Users/george/.config/nvim/lua/config/autocmds.lua:11: in function </Users/george/.config/nvim/lua/config/autocmds.lua:10>