Add filename/line functions to jq (fix #753)

This adds `input_filename` and `input_line_number` built-in functions
for use in, for example, `error` messages.

Example:

    $ printf '{"a":1}\n{"a":2}\n' > 4.json
    $ printf '{"a":"hello"}\n' > 5.json
    $ ./jq '{ "file":input_filename, "line":input_line_number, "value":.a }' 4.json 5.json
    {
      "file": "4.json",
      "line": 1,
      "value": 1
    }
    {
      "file": "4.json",
      "line": 2,
      "value": 2
    }
    {
      "file": "5.json",
      "line": 1,
      "value": "hello"
    }

jq.h

@@ -47,5 +47,7 @@ int jq_util_input_errors(jq_util_input_state);
 jv jq_util_input_next_input(jq_util_input_state);
 jv jq_util_input_next_input_cb(jq_state *, void *);
 jv jq_util_input_get_position(jq_state*);
+jv jq_util_input_get_current_filename(jq_state*);
+jv jq_util_input_get_current_line(jq_state*);
 
 #endif /* !JQ_H */