Support: 'Fetch >1000 documents with a POST query' using with_total option

[sengineer0]

Regarding to this issue: biothings#233
this PR try to support fetching >1000 documents with a POST query, by adding a new with_total option.

• if with_total option is not included in the POST body, or set to False, the result of query api will be same as the original behaviour: return a list of documents which match the query terms.
• if with_total option is set to True, the result will be reformated as a dict, which looks like this:

{
    'max_total': 100,  // the total documents matchs the query terms.
    'msg': '12 query terms return > 1000 hits, using from=1000 to retrieve the remaining hits',  // only included if there is at least one query term return greater than <size> of hits. the <size> value will be the options.size or default to 1000 if not set.
    'hits': [...]  // the actual response's documents
}