feat: remove fields not declared in template

[youen]

Problem

we can't use jl command to normalize a json stream with only wanted fields.

$ jl -t '{"name": "string"}' <<EOF
{"name": "Martin", "city": "London"}
EOF
{"name":"Martin","city":"London"}

Proposal

Add --strict flag (-s for short) to filter out not declared fields.

$ jl --strict -t '{"name": "string"}' <<EOF
{"name": "Martin", "city": "London"}
EOF
{"name":"Martin"}

[adrienaury]

--strict could be used to interrupt the pipeline on first error instead of logging an error and continuing to process lines.

I propose to use --discard or -d instead.