Ignore null member encoding

Add option to `encoder/2`, which enables ignoring `null` or `undefined` atoms to make small output json.
```erlang
  Encoder = jsonx:encoder([{r0, record_info(r0)}], [{ignore, [null]}]),
  <<"{}">> = Encoder({r0, null, null, null}).
```

Of course this encoding is irreversible, but sometimes size matters - the record can be rebuilt if json schema is  known, or record name is included inside the object.

```erlang
  [{<<"record_name">>, <<"r0">>}] = jsonx:decode(<<"{\"record_name\":\"r0\"}">>),
  {r0, null, null, null} = rebuild_r0([{<<"record_name">>, <<"r0">>}]).
```