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
    %

docs/content/3.manual/manual.yml

@@ -127,6 +127,14 @@ sections:
         variable. If you run jq with `--arg foo bar`, then `$foo` is
         available in the program and has the value `"bar"`.
 
+      * `--argfile name filename`:
+
+        This option passes the first value from the named file as a
+        value to the jq program as a predefined variable. If you run jq
+        with `--argfile foo bar`, then `$foo` is available in the
+        program and has the value resulting from parsing the content of
+        the file named `bar`.
+
   - title: Basic filters
     entries:
       - title: "`.`"