Add default font options

[etimberg]

Fixes #1954

Can set default font options by changing

Chart.defaults.global.defaultFontColor = '#666';
Chart.defaults.global.defaultFontFamily = "'Helvetica Neue', 'Helvetica', 'Arial', sans-serif";
Chart.defaults.global.defaultFontSize = 12;
Chart.defaults.global.defaultFontStyle = 'normal';

Some places do not use these options as their defaults are different than the global

• RadialLinear scale pointLabels fontSize defaults to 10
• Title font style is 'bold'
• Tooltip title font style is 'bold'
• Tooltip footer font style is 'bold'