Add possibility to have custom search on model

[RocKhalil]

You can now add a scope rails_admin_search that allows you to override the default search method. This will be useful for people who wants to search with joined model;

For example, if you're using globalize and the model's name column is translated, currently it fails to search for the correct one, but with pg_search and some minor basic querying with pg_search you'll be able to find your needed result.

Example of usage:

  pg_search_scope :rails_admin_search,
    :against => [:id],
    :associated_against => {
      translations: :name
    },
    using: {
      tsearch: {any_word: true,},
      trigram: {threshold: 0.1}
    }

[mshibuya]

The search method's name should be configurable via DSL.

[RocKhalil]

For the time being, it's good to have this one;
I'll work on making it configurable from Rails Admin's model config later on.

Let me know if you wish me to proceed with the second one before pulling.

-- Btw, this little fix saved me hours and hours of building a custom search page

[mshibuya]

Configurability is a key part of RailsAdmin, I don't merge this without it.
Also, having tests to make sure that this feature works is mandatory.

[RocKhalil]

@mshibuya I've moved the search_scope to model's config;
However, I'm not sure how to write tests for it since I use Guard instead of RSpec;
Here's how it can be used:

config.model Player do
  search_scope :my_search
end

class Player < ApplicationRecord
  scope :my_search, -> (keyword) { where(name: keyword) }
end

[mshibuya]

Based on your idea, 9abc7f9 is now in the master.
Thanks!

[RocKhalil]

@mshibuya excellent ! thanks :-)

[mshibuya]

Ah, I forgot to mention that I've made slight modification to the DSL.
I've updated the wiki accordingly.