Load elisp file before linting

Similar to #9 but for more complex situations, I'd like to be able to load a file with some configuration before linting happens. [Here](https://github.com/noctuid/annalist.el/blob/master/lint/lint-init.el) is an example. I'd also like to be able to fail CI runs if package-lint fails, but in order to do that, I need to be able to add advice to ignore unfixable errors.

If this seems reasonable, maybe look for a specific file name to avoid the need for user configuration (e.g. `lint-config.el`).