Opt-in support for including templates/*/node_modules in lessons

[AriPerkkio]

Is your feature request related to a problem?

In #198 we hard-coded node_modules to be excluded from templates. However in some cases users might want to have pre-optimized node_modules available in lessons. This featue should be opt-in, as most users will likely not need it and configuring such modules can be tricky.

Describe the solution you'd like.

Not yet sure which way would be better:

1. Define ignore patterns in templates:

---
type: lesson
template:
  name: default
  ignoredFiles: ["utils/**.js", "node_modules"]
---

---
type: lesson
template:
  name: default
  ignoredFiles: [] # Don't ignore any files, e.g. node_modules
---

However this can be tricky as <template-name>.template.json files are built in advance, before metadata is parsed.

2. Define ignore patterns in .tk-config.json

{
  "extends": "../default",
  "ignoredFiles": []
}

This would work well for <template-name>.template.json files too, but now exclusion is on template-level and cannot be defined per-lesson/part/chapter.

[Nemikolh]

I like the idea of defining the pattern in the config file a lot because it is more intuitive with inheritance and this seems to be a property of the template as opposed to be a configuration option of the part / chapter / lesson.