Allow option to force index scan

[bensheldon]

When querying on a table with a small number of records (like in development), the Postgres query-planner may ignore indexes in favor of sequential scans.

It would be beneficial to have an option to force Postgres to use indexes when performing the query to better simulate production behavior.

An example implementation is:

Wallet.where(base_currency: "USD").explain(analyze: true, indexscan: true)

would be equivalent to:

ActiveRecord::Base.connection.execute('SET enable_seqscan = OFF')
Wallet.where(base_currency: "USD").explain(analyze: true)
ActiveRecord::Base.connection.execute('SET enable_seqscan = ON')