Allow dynamic email subject

[tehfink]

It would be cool if the email subject could optionally use a template as the body does:

cmsplugin-contact-plus/cmsplugin_contact_plus/forms.py

Lines 173 to 177 in 58dbc76

	subject=instance.email_subject,
	body=render_to_string("cmsplugin_contact_plus/email.txt", {'data': self.cleaned_data,
	'ordered_data': ordered_dic_list,
	'instance': instance,
	}),