add start/stop options to branch ignore directives

[JeroMiya]

From this issue:
#15

This is a good start, but I have a use case where I need to ignore any branches between two directives, not just the 'next' one. This is for internal module definitions where the code always runs, but only once and never during a unit test. In my case, a code-gen tool is generating the nested internal modules and so I don't have a chance to add the directives for each nested module. Here is the output of the tool:

var app;
(function(app) {
  (function(common) {
    common.Foo = (function Foo() { this.foo = 'foo'; } return Foo; }());
   // need an option something like this:
   /* istanbul ignore start */
  })(app.common || (app.common = {}));
})(app || app = {}));
/* istanbul ignore end */

This should result in any code between the start and stop directives not to count towards statement or branch coverage percentages in the report and should be grayed out in the colorized report as in issue 15.

[gotwarlost]

Impossible to do this the way istanbul is designed since it looks at the AST and not at the source code. What I have implemented is pretty much what you can get.