Initial markdown support

Markdown doesnt have a language server, so do this manually
Closes #43

lua/symbols-outline/utils/lsp_utils.lua

@@ -7,4 +7,8 @@ function M.is_buf_attached_to_lsp(bufnr)
     return clients ~= nil and #clients > 0
 end
 
+function M.is_buf_markdown(bufnr)
+   return vim.api.nvim_buf_get_option(bufnr, 'ft') == 'markdown'
+end
+
 return M