feat: add education[].summary

[heitorPB]

fix #397

I was unsure about summary, I thought description also. I decided to go with summary because Education is something quite universal: area, studyType, and Institution describes almost everything; the only thing I miss about it is a field to describe briefly my Master's Thesis. Also, in the Work section, description is about the company and summary is about the job.

But I can change to description if you prefer.

Should I amend the sample.resume.json with this field?

[wilsonia]

I like this proposal. Only having the courses field to elaborate on education is too constraining.