Add option to support other file types

Would it be possible to add a configuration option that would allow the user to specify other filetypes that this plugin supports?

The elixir formatter now accepts plugins in the `.formatter.exs` file which allows `mix format` to support other file types.

See [this plugin](https://github.com/feliperenan/heex_formatter) for example that adds support for `heex` templates. It would be great if `vim-mix-format` could format these supported file types on save similar to .ex and .exs files.

Read the existing handling for .ex and .exs files in vim-mix-format, then compare it with the plugin configuration described in .formatter.exs. Done means users can configure additional types such as heex and have mix format run for them on save.

Written by the indexing model from the issue text.