Add an exclude/ignore option

[brunosxs]

When passing a folder, it would be useful to have a form to exclude a folder or files from the process.
Supose I am at the root of a godot project that has an addons folder that I want to exclude
'gdscript-formatter . -x addons'
or subfolder: 'addons/a_plugin_that_is_a_git_submodule'

[NathanLovato]

It makes sense! Thank you. I will also verify that it works with editorconfig as you should be able to mark excluded files and folders in team configuration (including exclusions from CI checks for example, if you use a third-party addon)