I got it working, thanks! I think I found a minor bug though. I could only get the --template flag to work when the file is in the current working directory. Subdirectories and absolute directories didn't work. I worked around this by simply cding into where my template was stored before running tinyfeed.
Even tinyfeed -i feeds.txt -o index.html -t ./template.html (with ./) results in:
fail to output HTML: fail to render HTML template: template: "./template.html" is an incomplete or empty template