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"
    }

docs/content/3.manual/manual.yml

@@ -2413,6 +2413,17 @@ sections:
           `["DEBUG:", <input-value>]` and prints that and a newline on
           stderr, compactly.  This may change in the future.
 
+      - title: "`input_filename`"
+        body: |
+
+          Returns the name of the file whose input is currently being
+          filtered.
+
+      - title: "`input_line_number`"
+        body: |
+
+          Returns the line number of the input currently being filtered.
+
   - title: Assignment
     body: |