[CLI] Allow additional customization file types

[peterjuras]

Summary

coat currently allows to customize a generated file by placing a file.ext-custom.js file next to the generated file that exports a function or content that is merged into the generated file.

It should be possible to have different customization file types based on the file type to make customization easier. For example, JSON files could allow for file.json-custom.json customization files that contain valid json that is merged into the output file. This would make it easier to quickly add or modify an existing property.

An example for json files:
.eslintrc

{
  "rules": {
    "no-param-reassign": "on"
  }
}

Current customization file:
.eslintrc-custom.js

module.exports = {
  "rules": {
    "no-param-reassign": "off"
  }
};

Proposed additional customization file possibility:
.eslintrc-custom.json

{
  "rules": {
    "no-param-reassign": "off"
  }
}