feat: add support for character spacing

[mbrgm]

This allows setting the (inter-)character spacing for the current font using pdf.SetCharSpacing(charSpacingInPt).

I tried to implement this as complete as possible and it works for my use case, but I am missing the detailed insight into this library to judge whether there is something missing. Feedback very much appreciated!

[oneplus1000]

Thanks for the pull request, I have merged successfully. But with a few code changes, I added that it can be modified to other units besides pt.

// SetCharSpacing : set the character spacing of the currently active font
func (gp *GoPdf) SetCharSpacing(charSpacing float64) error {
	gp.UnitsToPointsVar(&charSpacing)
	gp.curr.CharSpacing = charSpacing 
	return nil
}