Add custom types support

[astery]

Removes &to_string/1 in &number_to_delimited/1 because it's a reason for a failure of custom types what implement Number.Conversion conversion protocol but do not implement String.Chars (or implements it not the format that Decimal expects). And simply there is no need in it at all.

But the &number_to_phone/1 kept untouched because it heavily relies on string representation and if someone introduce a special type for the phone number representation it's doubtful that he would use any other functions from the package for that type.

[astery]

My initial need was to format monetary type from money package (I know it has it's own functions to format) via &number_to_delimited/2.

defimpl Number.Conversion, for: Money do
  @moduledoc false

  def to_float(value) do
    value
    |> Money.to_decimal()
    |> Decimal.to_float()
  end

  def to_decimal(value) do
    Money.to_decimal(value)
  end
end

Such call raises exception:

iex(2)> "1000" |> Money.parse!() |> Number.Delimit.number_to_delimited()
** (Decimal.Error) : number parsing syntax: "1 000,00"
    (decimal 2.1.1) lib/decimal.ex:1205: Decimal.new/1
    (number 1.0.3) lib/number/delimit.ex:104: Number.Delimit.number_to_delimited/2
    iex:2: (file)

This PR fixes this issue.