Add option to initialise swig

[thomas-lebeau]

Add a section init to be passed to swig.init(). İncluding root and we can add some (all?) other options for initialising swig. (At the same time it will be more clear, as root can be mistaken to src prefix path).

So we will have something like this:

swig: {
  dist: {
    cwd: 'source',
    dest: 'www/',
    src: ['**/*.swig'],
    init: {
      root: 'source/',
      allowErrors: false,
      autoescape: true,
      cache: true,
      encoding: 'utf8',
      filters: {},
      tags: {},
      extensions: {},
      tzOffset: 0
    }
    siteUrl: 'http://localhost:9000'
  }
}

[nickpack]

I just pushed 0.0.11 to npm, which contains this feature.