Make getopts optional

[zjp-CN]

This PR mainly gates getopts as a default feature, so if jsonxf is used as a library, getopts won't be included in user's Cargo.lock.

Besides I explicitly set edition to 2021, update the Cargo.lock and fix the README.

But I bump the jsonxf version to v0.1.2, so would you mind making a release on crates.io?

[blyxxyz]

Nice! This'll let us keep getopts out of our dependency tree as well.

I wouldn't bump the edition though, unless it's useful for code changes the only effect is to raise the MSRV.

[zjp-CN]

The only reason to set the edition is Cargo is emitting a warning for lack of edition field.

I don't mind setting it back to 2015.

[eirnym]

@gamache any news to include this patch?!