added ajsPath option

[dk1027]

Description

This PR adds an option ajsPath to override the default analytics.min.js location

Testing

Using default

snippet.max({})

The relevant bit of generated snippet:

  analytics.load = function(key, options){
    // Create an async script element based on your key.
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.async = true;
    script.src = 'https://cdn.segment.com/analytics.js/v1/' + key + '/analytics.min.js';

    // Insert our script next to the first script element.
    var first = document.getElementsByTagName('script')[0];
    first.parentNode.insertBefore(script, first);
    analytics._loadOptions = options;
  };

Setting ajsPath

snippet.max({ajsPath: '/foo/bar.min.js'})

  analytics.load = function(key, options){
    // Create an async script element based on your key.
    var script = document.createElement('script');
    script.type = 'text/javascript';
    script.async = true;
    script.src = 'https://cdn.segment.com/foo/bar.min.js';

    // Insert our script next to the first script element.
    var first = document.getElementsByTagName('script')[0];
    first.parentNode.insertBefore(script, first);
    analytics._loadOptions = options;
  };

[pooyaj]

@dk1027 is key a known variable?

[dk1027]

@pooyaj key is a variable in the scope of where settings.ajsPath is renderered: https://github.com/segmentio/snippet/pull/50/files#diff-7ca0ae40f9ef9562ed8a5fcdce4f82d2R71