[Accessibility] Add alt param to capturePlot()

[jooyoungseo]

Unlike plotTag(), capturePlot() does not have an alt param, which is a critical component for accessibility.

Please add this with default value as "Plot object" so that any NULL-alt plots can be also recognized by screen readers.

If possible, please add a condition to check whether current graph is ggplot2 object having alt attribute so that g$labels$alt can be passed to alt param when available.