Allow unwrap option to whitelist keys.

[rlburkes]

Recently this PR was merged to allow unwrapping of arrays. This capability is awesome and I am excited it landed. However, I recently had a use case that required me to unwrap only certain keys. These changes support whitelisting keys to unwrap. The option supports either an array of keys (symbols) to unwrap or a boolean value for wholesale unwrap/wrap behavior.

Unwrapping Arrays. You can specify an optional unwrap argument to modify the default Array
behavior. unwrap accepts a boolean flag (false by default) or an Array whitelist of keys to unwrap.

# Default Array behavior
Gyoku.xml({
  "foo" => [
    {:is => 'great' },
    {:is => 'awesome'}
  ]
})
# => "<foo><is>great</is></foo><foo><is>awesome</is></foo>"

# Unwrap Array behavior
Gyoku.xml({
  "foo" => [
    {:is => 'great' },
    {:is => 'awesome'}
  ]
}, unwrap: true)
# => "<foo><is>great</is><is>awesome</is></foo>"

# Unwrap Array, whitelist.
# foo is not unwrapped, bar is.
Gyoku.xml({
  "foo" => [
    {:is => 'great' },
    {:is => 'awesome'}
  ],
  "bar" => [
      {:is => 'rad' },
      {:is => 'cool'}
  ]
}, unwrap: [:bar])
# => "<foo><is>great</is></foo><foo><is>awesome</is></foo><bar><is>rad</is><is>cool</is></bar>"

Thanks for this awesome Gem. If you have any concerns with overloading this option I am open to any suggestions. I added some doc to the readme, I included a section on the new(er) key_converter functionality as it is something I found useful that wasn't included.

[tjarratt]

Thanks @rlburkes !