Add --argfile variant of --arg (issue #117)

This is useful when one has a database (in JSON form) to query using jq
input data.

    % echo '{"a":1, "c":5}' > db.json
    % echo '"c"'|./jq --argfile f /tmp/a '$f[.]'
    5
    % echo '"a"'|./jq --argfile f /tmp/a '$f[.]'
    1
    % echo '"b"'|./jq --argfile f /tmp/a '$f[.]'
    null
    %