allow to pass existing initials for names

[gr2m]

use case: multiple fields that might contain the same names. Example:

1. Project owner: 'John Doe'
2. Assigned: 'John Doe, Jane Dane'

In that case I'd like initials for 'John Doe' to be 'JD', and for 'John Doe, Jane Dane' to be 'JD, JDa' instead of 'JDo, JDa'

suggested API:

// pass existing initials for names
initials(['John Doe', 'Jane Dane'], {
  existing: { 'John Doe': 'JD' }
})
// ['JD', 'JDa']