Ability to pass custom labels for linkTitle

[bsaunders23]

Love the package so far, but would be great to be able to pass custom strings to append to the label (i.e. d.custlabel) when passing the links. Use case for me would be for including in addition to the total count the percent of records from a particular node moving down a given forward.

Current implementation

const fmt = d3.format('.3s')
function linkTitle(d) {
  const parts = []
  const sourceTitle = nodeTitle(d.source)
  const targetTitle = nodeTitle(d.target)
  const matTitle = d.type

  parts.push(`${sourceTitle} → ${targetTitle}`)
  if (matTitle) parts.push(matTitle)
  parts.push(fmt(d.value))
  return parts.join('\n')
}

Proposed

const fmt = d3.format('.3s')
function linkTitle(d) {
  const parts = []
  const sourceTitle = nodeTitle(d.source)
  const targetTitle = nodeTitle(d.target)
  const matTitle = d.type
  const custTitle = d.custlabel

  parts.push(`${sourceTitle} → ${targetTitle}`)
  if (matTitle) parts.push(matTitle)
  parts.push(fmt(d.value))
  if (custTitle) parts.push(custTitle)
  return parts.join('\n')
}