Add extensibility features

[cewald]

This might be the first step for extensibility features of #1.

I added a custom customFilters parameter to the buildQueryBodyFromSearchQuery method to add custom filters. This filters can be used like (important is the custom key to identify as custom filter):

searchQuery.applyFilter({ key: 'custom', value: 'filter_name', options: {  /* More complex options here */ } })

And the methods given to this customFilters parameter get a payload of parameters and work like:

({ name, options, queryChain, config }) => {
  // Do your custom filter stuff with the queryChain
  return queryChain
}

[cewald]

I will close this in favour of #6