Add support for tidbyt/hdk, removing dependency on Tidbyt cloud

[DouweM]

https://github.com/tidbyt/hdk is alternative firmware that doesn't depend on Tidbyt's servers and instead reads a WebP image from a (local) URL every 10s.

We could have use target-webp instead of target-tidbyt to write to files instead of the Tidbyt API, and implement our own app rotation in the server (#8) that loops through these rendered WebP files.

Since the Tidbyt API doesn't support max_age (tidbyt/pixlet#228) or app pinning, we could implement these ourselves.

This would not support closed-source official apps, but community apps can be run by Pixbyt as well.

[jdesai61]

This feature will be greatly appreciated! I am using MatrixPortal S3 board running HDK port from https://github.com/mrjefftang
And would love to be able to manage push to HDKs running on non-Tidbyt devices.