Switch img alt parsing to on by default

[snarfed]

It's been over 3 yrs since this was officially incorporated into the spec, 4-5 yrs since #83 and kartikprabhu#56. img alt parsing is pretty standard and expected now, and it trips people up when they don't know they need to turn it on with img_with_alt=True. It's probably time to drop the feature flag, or at least switch it to on by default!

cc @ludovicchabant

[gRegorLove]

Agreed. img-alt parsing has been live in php-mf2 since 2022-02-09.

[capjamesg]

I have filed a PR to implement this (PR #184). @snarfed I have tagged you as a reviewer.

[sknebel]

For context, this has gotten "stuck" because it's a breaking change and there were more things on the list before doing a major version change before this all fell asleep. I'd propose to do the change Python3 only and then make a major version jump.

[capjamesg]

I agree. Python 2.7 and 3.6 deprecation, and the corresponding test suite environment fixes, is at the top of the TODO list.

[capjamesg]

The PR implementing this (#184) has been merged and will be included in the next release.