Tstickers

- Fix https://github.com/FHPythonUtils/SigStickers/issues/1
- More meaningful error messages

- Implement action='extend' for pre 3.7 eg. `python3 -m tstickers -p pack1 pack2 -p pack3`

- Update function names and docs

- Added caching functionality using requests_cache and to the converter -
output cache hit/miss to stdout for converter

- Use `asyncio.get_event_loop().run_until_complete` in place of `asyncio.run` for compat
with pyrlottie 2021.1
- Marginal performance improvements with pyrlottie 2021.1 (~3% so may be a fluke?)

txt
Performance testing with https://t.me/addstickers/DonutTheDog on:
OS: Windows 10 (2021/10/03)
CPU: Intel(R) Core(TM) i7-10510U CPU 1.80GHz
RAM: 16gb

using pyrlottie (lottie2gif.exe + gif2webp.exe)
~85s (frameskip=0, scale=1) (-5s)
~47s (frameskip=1, scale=1) (0s)
~33s (frameskip=2, scale=1) (-1s)

- Produce pngs for animated stickers as in SigStickers
- Tidy up