Meta information for optional fields

[tedtoal]

In the VCF spec v4.3 5-Apr-2018, the "Meta-information-lines" section says that optional fields must be stored as strings even for numeric values. I suggest that this be changed to read that optional fields "must either have a ##META line describing them (preferred), or must be stored as strings even for numeric values." This would jive with the use of ##META lines to describe optional fields in the section "Sample field format".