(piqi convert, ocaml) Add new options for converting from/to Piq format

--piq-frameless-output true|false print a frame around a single output Piq object (default=false)
--piq-frameless-input true|false expect a frame around a single input Piq object (default=true)
--piq-relaxed-input true|false parse Piq format using "relaxed" mode (default=false)
(piqi tools, ocaml) Rename json_omit_null_fields -> json_omit_missing_fields

... to better reflect the purpose of the conversion option.

The change is backward compatible as the old name is still accepted.