Initial markdown support

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

lua/symbols-outline/config.lua

@@ -65,9 +65,7 @@ function M.get_position_navigation_direction()
     end
 end
 
-function M.get_width_percentage()
-    return M.options.width / 100
-end
+function M.get_width_percentage() return M.options.width / 100 end
 
 function M.get_split_command()
     if M.options.position == 'left' then
@@ -90,6 +88,7 @@ end
 
 function M.is_client_blacklisted(client_id)
     local client = vim.lsp.get_client_by_id(client_id)
+    if not client then return false end
     return has_value(M.options.lsp_blacklist, client.name)
 end