Added ability to set uuid as item_id type while generating migration

[Nuzair46]

Added option --uuid when generating new migration. This can be used to set the type on item_id column to uuid.

Use case:

• If a user uses paper_trail on a database that uses uuid as primary key, there is a chance where item_id becomes 0 causing the versioning to not work. Changing the type of item_id from bigint to string fixes this issue.
• This PR makes it possible for the user to use uuid while generating the migration.

bin/rails generate paper_trail:install [--uuid]

Thank you for your contribution!

Check the following boxes:

• Wrote good commit messages.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new
  code introduces user-observable changes.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[jaredbeck]

Thanks Nuzair! Generally this looks good, just a few minor comments.

Also, please update the install documentation in the README.

[Nuzair46]

@jaredbeck , Suggested changes are made. Please review. Thanks.

closes #1373

[jaredbeck]

Thanks for the contribution!