Support for Amazon Echo Show

[RalfEggert]

The Amazon Echo Show needs to be supported. Especially the template directives. See these links for details:

• https://developer.amazon.com/de/blogs/alexa/post/12826e9e-e06a-4ab4-a583-8e074709a9f3/how-to-build-alexa-skills-for-echo-show1
• https://developer.amazon.com/de/docs/custom-skills/build-skills-for-echo-show.html
• https://developer.amazon.com/de/docs/custom-skills/best-practices-for-echo-show-skills.html

[RalfEggert]

Added response directives for body templates with MR #29

[RalfEggert]

Added request context display object with MR #30