lunr = function (config) {
    var idx = new lunr.Index

    idx.pipeline.add(
        lunr.trimmer,
        lunr.stopWordFilter,
        lunr.stemmer
    )

    if (config) config.call(idx, idx)

    return idx
  }
Convenience function for instantiating a new lunr index and configuring it
with the default pipeline functions and the passed config function.

When using this convenience function a new index will be created with the
following functions already in the pipeline:

lunr.StopWordFilter - filters out any stop words before they enter the
index

lunr.stemmer - stems the tokens before entering the index.

Example:

    var idx = lunr(function () {
      this.field('title', 10)
      this.field('tags', 100)
      this.field('body')

      this.ref('cid')

      this.pipeline.add(function () {
        // some custom pipeline function
      })

    })

@param {Function} config A function that will be called with the new instance
of the lunr.Index as both its context and first parameter. It can be used to
customize the instance of new lunr.Index.
@namespace
@module
@returns {lunr.Index}