Opt in to register aucmd for auto format and ast check.

[Tiseno]

From #51 (comment)

The g: and Format should also probably be renamed to something like FormatAndAstCheck.

Also from the discussion: this whole plugin should probably be split into two different plugins, one for filetype detection and syntax, which should be included in vim, and the other one for things like formatting and linting, like how the go plugin was split: https://github.com/google/vim-ft-go and https://github.com/fatih/vim-go

Or probably it is just the case that vim should stop upstreaming everything from this plugin, and just take the bits it should (ft detection+syntax+indent+compiler definition).

[Tiseno]

PR for removing this from vim vim/vim#13803
So should probably close this if it gets merged.