Allow nav category icons to be specified as a path like nav-route icons

[TheOtherDude]

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

• [ ] #none# - documentation fixes and/or test additions
• [ ] #patch# - backwards-compatible bug fix
• [x] #minor# - adding functionality in a backwards-compatible manner
• [ ] #major# - incompatible API change

This is the same as PR #221 except for category icons instead of route icons.

CHANGELOG

• Added ability to specify category icons as urls, mirroring the functionality already implement for route icons.

[coveralls]

Changes Unknown when pulling f40857be89463a6375c9f963b93bda1869c17046 on TheOtherDude:category-icon-urls into on ciena-frost:master.

[notmessenger]

👍