Detect input format based on file name extension

[ryenus]

When the inputFormat is not specified, instead of just assume the input format to be "yaml", we can also look at the input filename, more specifically the file extension, to deduce the input format.

This Fixes #1440.

[mikefarah]

Looks really good - could you add some acceptance tests (see acceptance_tests folder) to test that the detection works for the various file types?

[ryenus]

@mikefarah Thank you! I added a test, actually just copied but removing the explicit -p args.
Hopefully it works as expected. Just a humble attempt as a rare-time gopher, Please feel free to amend :-)

[ryenus]

@mikefarah, any idea why testLeadingSeparatorDoesNotBreakCommentsOnOtherFiles is broken? Could you please take a look?

shunit2:ERROR testLeadingSeparatorDoesNotBreakCommentsOnOtherFiles() returned non-zero return code.
ASSERT:expected:<# a1
a: 1
# a2

# b1
b: 2
# b2> but was:<# a1
a: 1
# a2

b: 2
# b2>

[mikefarah]

Looks like the #b1 comment disappeared 🤔 not sure why that would happen

[hubchub]

I don't think this belongs in a v4.x release.
It is a nice feature and all but it totally breaks backwards compatibility.

[ryenus]

@hubchub, no, detecting input format based on the filename is not really breaking, as it works only when the input format is not specified. Meanwhile the explicitly set input format is always respected, just like how it behaved in the past. If you meant the change related to the default output format, that's a separate topic.