Enhancement: Auto-convert ``PlotLines.value`` from ``datetime`` to POSIX timestamps

[hcpchris]

As identified in #57 , and in alignment with the Highcharts (JS) API, the options.axes.plot_bands.PlotLine.value property expects a numeric value. Currently, if a Python datetime is supplied, it fails on type validation. However, if the datetime is converted to a POSIX timestamp it works correctly.

Therefore, it would be good for the PlotLine.value property to auto-convert datetime.datetime values to POSIX timestamps using the .timestamp() method.