Support tax_query filter (added in WP-API/WP-API#2748) - #243

This PR provides a convenience method for setting the `tax_query` filter:

```js
// Return only posts in category 29
site.posts().taxQuery({
  taxonomy: 'category',
  terms: [ 29 ]
})...
```

Note that if a relation is specified, numeric indices must be provided:

```js
// Return posts in (cats 49 or 29) && in tag 79 && NOT in tag 106
const request = site.posts().taxQuery({
  relation: 'AND',
  '0': {
    taxonomy: 'category',
    terms: [ 49, 29 ]
  },
  '1': {
    taxonomy: 'post_tag',
    // field: 'slug',
    terms: [ 79 ]
  },
  '2': {
    taxonomy: 'post_tag',
    terms: [ 106 ],
    operator: 'NOT IN'
  }
});
```

_If_ "OR" can be used as the topmost relation, an array of tax_query
objects can be provided to the filter method instead; however, any
nested queries must use array indices because the array -> object
parsing is not recursive.

```js
// Implicit OR: this query is, all posts with
// (tag #93 && category #49) || (tag #81 && not tag #116)
site.posts().taxQuery([{
  relation: 'AND',
  '0': {
    taxonomy: 'category',
    terms: [49]
  },
  '1': {
    taxonomy: 'post_tag',
    terms: [93]
  }
}, {
  relation: 'AND',
  '0': {
    taxonomy: 'post_tag',
    terms: [81]
  },
  '1': {
    taxonomy: 'post_tag',
    terms: 116,
    operator: 'NOT IN'
  }
}]);
```