Refactored AttrTree repr to support custom node repr
Refactored AttrTree repr to support custom node repr

ioam/holoviews,mjabri/holoviews,ioam/holoviews,mjabri/holoviews,mjabri/holoviews,ioam/holoviews,vascotenner/holoviews,basnijholt/holoviews,basnijholt/holoviews,vascotenner/holoviews,vascotenner/holoviews,basnijholt/holoviews