User config is ignored if `auto_cmd` is set to false

[heygarrett]

The file types and buffer types to exclude are ignored when auto_cmd is set to false. However, I only disabled the autocmd so I could call guess-indent from my own autocmd, to allow for more control over when it's run relative to other indentation-related configuration.

Currently I'm manually calling require("guess-indent").set_from_buffer(true) from my own autocmd, but it would be great if there was a command, or perhaps an argument that could be passed to :GuessIndent, that would respect the user config regardless of what auto_cmd is set to.