New insert favorite gist command

[k-kuroguro]

Description

Add new command to insert starred gist.

Related Issue

142

Motivation and Context

I use gist to manage my snippets, and it would be great if I could insert starred gist as well as my own.

How Has This Been Tested?

I manually tested that the command can be called from the command palette, that quickpick shows all starred gists, and that the editor inserts the texts. And, I add test script with reference to the insert command.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[coveralls]

Coverage increased (+0.4%) to 83.959% when pulling 3834936dd178a13f94de4c4b0d0af72824419887 on k-kuroguro:insert-favorite-gist into 0484d03a7eea2603ffa5226243d6a5895b1ab593 on kenhowardpdx:develop.