Add isLoading flag to show loading icon

[anhuynh]

Overview

Add an optional isLoading flag to show a loading icon in the middle of the list

Does this PR close an existing issue?

No PR should be opened without opening an issue first. Any change needs to be discussed before proceeding.

Summary

Issue Number(s)

Which issue(s) does this PR address?

Put Closes #XXXX below to auto-close the issue that this PR addresses:

• Closes #

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Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have evaluated if the README.md documentation needs to be updated
• I have evaluated if the /tests/dummy/ app needs to be modified
• I have evaluated if DocBlock headers needed to be added or updated
• I have verified that lint and tests pass locally with my changes
• If a fork of a dependent package had to be made to address the issue this PR closes:
  • I noted in the fork's README.md the reason the fork was created
  • I have opened an upstream issue detailing what was deficient about the dependency
  • I have opened an upstream PR addressing this deficiency
  • I have opened an issue in this repository to track this PR and schedule the removal of the usage of the fork

Semver

This project uses semver, please check the scope of this PR:

• #none#
• #patch#
• #minor#
• #major#

Examples:

• NONE
  • README.md changes
  • test additions
  • changes to files that are not used by a consuming application (.travis.yml, .gitignore, etc)
• PATCH
  • backwards-compatible bug fix
    • nothing about how to use the code has changed
    • nothing about the outcome of the code has changed (though it likely corrected it)
  • changes to demo app (/tests/dummy/)
• MINOR
  • adding functionality in a backwards-compatible manner
    • nothing about how used to use the code has changed but using it in a new way will do new things
    • nothing about the outcome of the code has changed without having to first use it in a new way
    • addition of new CSS selectors
    • addition of new ember-hook selectors
• MAJOR
  • incompatible API change
    • using the code how used to will cease working
    • using the code how used to will have a different outcome
    • any changes to CSS selector names
    • any removal of CSS selectors
    • any changes to ember-hook selectors
    • possibly changes to test helpers (depends on the changes made)
  • any changes to the dependencies entry in the package.json file

CHANGELOG

• Add optional isLoading flag to show loading indicator