Make use of font weight settings

[agilob]

Backwards incompatible change as fontWeight used to be String now it's a class object

closes #351

I have read contributing guidelines, external contributions unfortunately cannot be taken at this time. but still giving it a shot. I would rather you merge it then me fork it and release on pubdev as a fork with one fix.

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[agilob]

@googlebot I signed it!

[googlebot]

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[winksaville]

There is a fork of charts at exafree/charts_cf that accepts pull requests.

[stuartmorgan]

This project is no longer maintained, and cannot accept any PRs. Community members interested in collaborating on a community-maintained fork can coordinate in this issue.