Add Blocks support

[aderyabin]

I suggest to handle values using blocks.
Less words - more examples:

element :title do  |title|
  "#{title}!!!"
end

attribute :id do |id|
  id.to_i
end

element :link, value: :foo do |link|
  link.downcase
end

[krasnoukhov]

Thanks! This looks pretty convenient. Merging ✨

[krasnoukhov]

Also, @prodis you might be interested in this. Now your boolean use case implementation (see #58) can be much nicer:

require 'sax-machine'
require 'wannabe_bool'

class Fake
  include SAXMachine
  element :name
  element :main, &:to_b
  element :published, &:to_b
end

xml = '<fake><name>Prodis</name><main>true</main><published>false</published></fake>'

fake = Fake.parse(xml)
puts fake.main
puts fake.main.class

Output:

true
TrueClass

Do you mind giving it a try on master? Thanks!