Allow synchronous call tags to work inside redirects

[kirsle]

This change makes it possible to include synchronous object macro calls inside of redirects (both the command and inline form).

This fixes #201.

Example from the unit test:

> object echo javascript
  var message = args.join(" ");
  return message;
< object

+ hello bot
- Hello human.

+ redirect to *
@ <call>echo <star></call>

+ inline to *
- "<sentence>": {@ <call>echo <star></call>}

This is similar to the bug fix in #111 when object macros were fixed for conditional commands. It only supports synchronous macros (ones which return a string reply, not a promise) for the same reason.

In order to avoid regular expression problems with the {@...} syntax's curly brackets conflicting with those from the mangled {__call_arg__} types of tags, I changed the delimiter of the mangled call tags to use the 'LEFT-POINTING DOUBLE ANGLE QUOTATION MARK' (U+00AB) and 'RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK' (U+00BB)' symbols: « and ». I chose these symbols so that if the call tag fails to parse properly (as it did in the original bug), the resulting text contains printable symbols. « and » are not used by anything else in RiveScript.