Feature/is main query

[lipemat]

Support using $query->is_main_query() when ES_WP_Query is replacing WP_Query

Automatically passed a query_var which is later used within an override method of is_main_query during pre_get_posts actions.

Fixes #38

[WPprodigy]

I like this approach. What do you think about using es_is_main_query instead, mainly to go a little bit out of the way to avoid potential conflicts?

With some tests, this should be a safe addition and a good solution for 38.

[lipemat]

I like this approach. What do you think about using es_is_main_query instead, mainly to go a little bit out of the way to avoid potential conflicts?

That is a great suggestion! I have updated the code.

[rebeccahum]

Tested & works great!