Allow specifying custom twig instance

[iby]

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Thanks for the great plugin! Small feature request. I have a pre-configured twig instance with custom extensions. Ideally I want to reuse that rather than configuring a new one via the plugin.

var twig = require('twig');
var gulpTwig = require('gulp-twig');

gulpTwig({twig:twig, data: …});

This is a similar feature request to one found in gulp-sequence – it allows to reuse an existing gulp instance instead of requiring it's own.