Test template filters

[orsinium]

1. Make template filters kind of type-safe. leave type reflection to the template engine and accept already the right type. Also, it helps to correctly match the number of arguments, making error messages in that case user-friendly.
2. Add unit tests for happy-path for custom filters.
3. Do not suppress errors from filters, making them propagate up into the template engine. The engine gracefully handles it and returns as the second argument from Template.Execute method indicating template execution failure.

Thank you for the amazing project! I'm migrating from IFTTT :)

[coveralls]

Coverage increased (+2.6%) to 71.515% when pulling 3cb789a on orsinium-forks:improve-templates into d22455f on muesli:master.

[orsinium]

CI fails not because of my changes, I'm sure. Is it flaky? 🤔

[rocket357]

Yes. I put in a PR for docker and CI failed for windows! The docker file isnt (to my knowledge) used or touched in the CI builds. So I would assume CI is flakey.

…

On Fri, Oct 2, 2020, 08:07 Gram ***@***.***> wrote: CI fails not because of my changes, I'm sure. Is it flaky? 🤔 — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#333 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAGQUQWNHHVOTKEJS2D7GPDSIXGB7ANCNFSM4SBVIMNA> .

[muesli]

Just a heads up: @penguwin is already working on fixing the Windows builds.

[muesli]

Great contribution, thank you!