Add new parameter for including extra plugin in build

[Dumluregn]

To test:

• build a ckbuilder.jar file (requires ant installed).
• go to ckeditor4-presets repo.
• change build.sh file to use the local ckbuilder.jar file.

Closes #29.

[Dumluregn]

There are two things I'd like to mention:

1. Serving multiple plugins could be done easier with string.prototype.indexOf() method, but then it would misfire with plugin names being part of other plugin names, e.g. autolink plugin would also include link plugin. Therefore I went with splitting the names after each comma, which in turn required writing an alias for the array.prototype.includes() method, but it works fine.
2. There is an edge case where we could want to include a plugin with some plugin dependencies, but currently those deps won't be included. It can be done since the logic for extracting required plugins is already there, but it's still work, so please decide if we want to cover it or not.