Added support for subqueries in Where and Having clauses

[ManReinsp]

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What did this pull request do?

Allows developers to use subqueries in the Where and Having clause.
A test is available in the main_test.go:TestQueryBuilderSubselectInWhere function.

Example:

DB.Select("*").Where("age >= ?", DB.
		Select("AVG(age)").Table("users").Subquery()).Find(&users)

[gerhardgruber]

@ManReinsp eprx -> expr

[jinzhu]

Hi @ManReinsp

Thank you for your PR and sorry for the delay, I was thinking how to merge those changes w/o replaceParameterPlaceholderLiteral.

Then I have made QueryExpr method (still thinking a better name...) mostly based on your code and test cases.

Thank you.