Add keywords to work items

[oli77za]

I understand that this can be redundant with highlights, however, I think it would help if, for every work experience I could add some tags that are relative to that experience (I am a developer, so I would say Java8 Python React, and the next experience is AWS etc....)

In this way, the resume reader can identify quickly which experiences are relevant to a job position

Thoughts?

[pflorek]

Maybe we put keyword also to the other sections

There's another PR https://github.com/jsonresume/resume-schema/issues/420