Add mix ecto.query

[josevalim]

This is inspired by rails query:

• Add a read_only: true option to Repo.transaction (databases that do not support it should raise)
• Add mix ecto.query "from(p in Post, where: ...)" which will:
  • Load the current dot iex if any
  • import Ecto.Query
  • Start a read-only transaction
  • Eval the code
  • And pass it to Repo.all using the default repo
• Print the results using inspect(limit: :infinity) but no more than 100 entries (controlled by --limit flag)

Question: do we print the high-level schema data (if yes, then we should remove all association fields) or the {MyXQL,Postgrex,...}.Result struct? Rails prints the result struct. If we go with the result, then this belongs in ecto_sql (which could be beneficial as we could support a --sql flag too).