allow files to be used as HMAC secrets

[jbg]

This PR resolves the issue I reported in #109, so that HMAC secrets can be read from a file. Previously, if one attempted to read from a file by passing @filename (as the --help text suggests), the literal string @filename was used as the secret instead.

As an aside, anyone who has used this tool with HMAC JWTs must have quite brute-forceable secrets, since they likely contain only printable characters.

[mike-engel]

Thanks for this @jbg! Could you also add some tests for slurping files? If you're unsure how to proceed, let me know and I can try to describe it here

[jbg]

You mean just testing the combination of file token and HMAC? There are already two tests of the file slurping code at the end of jwt-cli.rs

[mike-engel]

Correct, just for HMAC file tokens. At some point, I'd like to make the file slurping work for everything, but until then, I'd like to add a test for HMAC since they don't share the same code and to make sure it doesn't break in the future

[jbg]

Got it! Will take care of this a little later. Thanks for the clarification!

[mike-engel]

Hi @jbg, I wanted to check in to see if you've had time to add some tests, or if you need any help. Thanks!

[mike-engel]

Closing in favor of #130