Enable using the chartjs subpackage separately

[marianogappa]

A wealth of use cases would be enabled if charts could be made programatically instead of using the CLI tool proper, e.g. #16

Required changes are minimal:

• Calculating minFSS and maxFSS within the subpackage rather than receiving them
• Producing the full HTML/JS/CSS should be parameterised and implemented within the subpackage
• Examples, better godoc and a README in the subpackage

[marianogappa]

Merged in #40.
Please review:

• https://godoc.org/github.com/marianogappa/chart/chartjs
• https://github.com/marianogappa/chart/tree/master/chartjs