travis-ci: build on R16B01, R16B02 OTP releases
Add new options for converting from/to Piq format

piq_frameless_output

     Print a frame (i.e. :<typename> []) around a single output Piq object
     (default=false)

piq_frameless_input

     Expect a frame around a single input Piq object (default=false)

piq_relaxed_parsing

     Parse Piq format using "relaxed" mode (default=false);

     For instance, when set to `true`, single-word string literals don't have
     to be quoted
Rename conversion option: json_omit_null_fields -> json_omit_missing_fields

... to better reflect the purpose.

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