Add command to explain piece of code in a PR

[scastlara]

Is your feature request related to a problem? Please describe.
I want to ask lgtm for clarifications on existing prs.

Describe the solution you'd like
Add a new command lgtm explain that receives:

• PR url
• File path
• Line number

And it adds an answer to the original comment explaining the piece of code in question.

• For GitHub, this can be done (I think) with

on:
  pull_request_review_comment:
    types: [created]

// and then
// echo "File: ${{ github.event.comment.path }}"
// echo "Line: ${{ github.event.comment.line }}"

• For GitLab, it is much more complicated. I cannot see any when or rule for comments, so we would need a webhook. Then you would also need a backend for receiving the webhooks 🤯

For now, we can create the command and add the GitHub workflow. We can leave GitLab out for the time being...