Escape special characters in append unique actions

[crutchcorn]

This PR was originally made by @soulfresh in the node-plop dedicated repo and moved over by hand to me: https://github.com/plopjs/node-plop/pull/207

What does this PR do?

Updates the append action to escape special characters in the replacement text when using the unique configuration.

Resolves https://github.com/plopjs/plop/issues/275.

Here is the failing scenario:

// index.js template
/* PLOP_INJECT_EXPORT */
export * from './foo';

// plopfile.js action definition
ACTION: {
  type: 'append',
  data: {},
  path: indexFilePath,
  template: "export * from './foo';",
  pattern: '/* PLOP_INJECT_EXPORT */',
  unique: true,
}

// index.js file result
/* PLOP_INJECT_EXPORT */
export * from './foo';
export * from './foo';

This PR ensures the duplicates don't end up in the output.

Relates to https://github.com/plopjs/plop/issues/174