Add marsman/boostrap/default styles for simple form

[xredo]

Closes #27

[fjsanpedro]

It is not a big deal but, instead having the same behavior duplicated why not gather it:

`
def run_generators
case ask_framework_for_forms
when 'marsman'
@template.initializer 'simple_form.rb', simple_form_marsman_template
when 'bootstrap'
@template.generate 'simple_form:install --bootstrap'
else
@template.generate 'simple_form:install'
end
end

def add_sample_i18n
  @template.run 'rm config/locales/simple_form.en.yml'
  @template.append_to_file 'config/locales/en.yml', simple_form_locale_sample
end

def ask_framework_for_forms
  @template.ask('What framework do you want to use for your forms?',
                limited_to: %w(marsman bootstrap default))
end

def simple_form_marsman_template
  simple_form_sample('marsmab.rb')
end

def simple_form_locale_sample
  simple_form_sample('en.yml'')
end

def simple_form_sample(filename)
  File.read(File.join(File.dirname(__FILE__), 'simple_form', filename))
end

`

My 2 cents

[xredo]

What do you think @fjsanpedro? I'm only using one method, instead of 2, but having the behaviour unified was necessary, as you suggested!

[fjsanpedro]

Great, ready to 🚀