Add padding

[emeeks]

Best practices and guides often suggest, for instance, that a line chart shouldn't extend to the very bottom or very top of a chart, so expose some mechanism that allows you to give pixels so expose a padding prop on frames that does the following:

1 is padding in pixels. <1 is padding in percent. Padding can be a number or an object with left,right,top,bottom.

Honor an "auto" mode that tries to derive smart padding from data

[emeeks]

Duplicate of #444