Allow random extensions

[chevdor]

We may want to pass a custom extension such as .foobar.
In that case, we could pass an extra flag to provide the format (such as --yaml) or be smart and guess.

[alerque]

This would also be useful for being able handle process substitution without so much song and dance to get extensions sorted:

TMPSUFFIX=.yml
tera -t template <(command_that_generates_context)
unset TMPSUFFIX

Could be:

tera -t template --yaml <(command_that_generates_context)

Personally I would suggest not trying to be smart and guess the type if it isn't noted as an extension. YAML is a disaster to detect because it's technically a superset of JSON, so most JSON is valid YAML and YAML can be made to look very much like JSON but actually work the same way a JSON parser would handle it.

[chevdor]

I agree, I am only using the detection based on the extension so spare the user yet another flag but the current rules are rather tight.