feature: extract! api for getting attributes from object

[kortirso]

discussed in #30

[jho406]

This is a great add for users coming from jbuilder. A bit of parity helps.

json.extract! user, :id, :email, :name

There is just one edge case:

json.extract! user, :id, :email, :full_name

When the attribute is full_name, but as props_template doesn't autocamelize keys, a dev expecting fullName, actually gets full_name. What are your thoughts on this syntax to allow for manual setting of they key:

json.extract! user, :id, :email, [:full_name, :fullName], :city, [:phone_number, :phoneNumber], :country

[kortirso]

When the attribute is full_name, but as props_template doesn't autocamelize keys, a dev expecting fullName, actually gets full_name. What are your thoughts on this syntax to allow for manual setting of they key:

json.extract! user, :id, :email, [:full_name, :fullName], :city, [:phone_number, :phoneNumber], :country

I thought about passing block if key needs to be transformed, but this is better

[jho406]

Looks great! Thanks @kortirso !