[PATCH] few fts functions for jsonb

Hi all

I would like to propose patch with a set of new small functions for fts in
case of
jsonb data type:

* to_tsvector(config, jsonb) - make a tsvector from all string values and
elements of jsonb object. To prevent the situation, when tsquery can find
a
phrase consisting of lexemes from two different values/elements, this
function will add an increment to position of each lexeme from every new
value/element.

* ts_headline(config, jsonb, tsquery, options) - generate a headline
directly
from jsonb object

Here are the examples how they work:

```
=# select to_tsvector('{"a": "aaa bbb", "b": ["ccc ddd"], "c": {"d": "eee
fff"}}'::jsonb);
to_tsvector
-------------------------------------------------
'aaa':1 'bbb':2 'ccc':4 'ddd':5 'eee':7 'fff':8
(1 row)

=# select ts_headline('english', '{"a": "aaa bbb", "b": {"c": "ccc
ddd"}}'::jsonb, tsquery('bbb & ddd & hhh'), 'StartSel = <, StopSel = >');
ts_headline
----------------------
aaa <bbb> ccc <ddd>
(1 row)
```

Any comments or suggestions?