Add join support

[paulcsmith]

This will be fairly difficult to tackle, but I'm happy to help. Feel free to reach out in https://gitter.im/luckyframework/Lobby for help

When you define an association with has_many or belongs_to it should add a number of fields to the BaseQuery

class Comment < BaseModel
  table :comments do
    belongs_to post : Post
  end
end

Comment::BaseQuery.new.join_post 
# or
.inner_join_post
.left_join_post

This should also work for has_many associations. This isn't that helpful on its own, but it will be required for #61 and #59

This will require:

• Adding a @joins to the QueryBuilder

  lucky_record/src/lucky_record/query_builder.cr

  Line 5 in 8607673

  @wheres = [] of LuckyRecord::Where::SqlClause

  to hold all the join clauses
• Add the join clauses the sql_statements method. This wll work similarly to how the where_sql is generated.

  lucky_record/src/lucky_record/query_builder.cr

  Line 51 in 8607673

  [wheres_sql, limit_sql, offset_sql, order_sql]

• Keep track of associations that were added, similar to what was done with FIELDS

  lucky_record/src/lucky_record/model.cr

  Line 5 in 8607673

  FIELDS = [] of {name: Symbol, type: Object, nilable: Bool, autogenerated: Bool}

• Pass them to the base query template:

  lucky_record/src/lucky_record/model.cr

  Lines 64 to 66 in 8607673

  	macro setup_base_query_class(table_name)
  	LuckyRecord::BaseQueryTemplate.setup({{ @type }}, {{ FIELDS }}, {{ table_name }})
  	end

• Create methods in the BaseQueryTemplate for each of the associations: https://github.com/luckyframework/lucky_record/blob/master/src/lucky_record/base_query_template.cr