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"
    }

builtin.c

@@ -1126,6 +1126,12 @@ static jv f_now(jq_state *jq, jv a) {
 }
 #endif
 
+static jv f_current_filename(jq_state *jq) {
+  return jq_util_input_get_current_filename(jq);
+}
+static jv f_current_line(jq_state *jq) {
+  return jq_util_input_get_current_line(jq);
+}
 
 #define LIBM_DD(name) \
   {(cfunction_ptr)f_ ## name, "_" #name, 1},
@@ -1188,6 +1194,8 @@ static const struct cfunction function_list[] = {
   {(cfunction_ptr)f_mktime, "mktime", 1},
   {(cfunction_ptr)f_gmtime, "gmtime", 1},
   {(cfunction_ptr)f_now, "now", 1},
+  {(cfunction_ptr)f_current_filename, "input_filename", 1},
+  {(cfunction_ptr)f_current_line, "input_line_number", 1},
 };
 #undef LIBM_DD