Command processor: composition commands

[alyu201]

Is your feature request related to a problem? Please describe.
All composition commands need to be implemented.

Describe the solution you'd like
The voice inputs should be defined and mapped to associated code snippets.

Checklist for the issue creation
Current commands to implement:

• Add element with no attributes:
  • <div> = "container"
  • <p> = "paragraph"
  • <h1-6> = "heading 1-6"
  • <ul><li> = "unordered list"
  • <ol><li> = "ordered list"
• Add element with one attribute (implicitly requires "add text" command):
  • <a href> = "anchor"
  • <img src> = "image"