Make webpack plugin being able to run inside the ckeditor5-* package

[ma2ciek]

Suggested merge commit message (convention)

Fix: Made webpack plugin being able to run inside the ckeditor5-* package. Closes #302.

[ma2ciek]

I'm considering moving functions, that are inside replacetcalls to TranslationService class for being able to test them.

[pomek]

Would be good to have at least one test which checks whether the paths are correct. But I don't know how much changes it requires.

[ma2ciek]

@pomek. I'm working on the ckeditor/ckeditor5#624, so I'll be probably rewriting those part to make TranslationService and the plugin's core part more generic. So I think that we can wait for the incoming changes before rewriting that part for tests. I'll keep that in mind.

[pomek]

I will check whether builds work. If nothing will crash, I'll merge.

[pomek]

Error: original.line and original.column are not numbers -- you probably meant to omit the original mapping entirely and only map the generated position. If so, pass null for the original mapping instead of an object with empty or null values.

But it isn't a part of this ticket.