Add flex properties to safelist

[miguelperez]

I submitted the question as an issue, but then figured it is better to submit a PR. #197

https://css-tricks.com/snippets/css/a-guide-to-flexbox/

[flavorjones]

@miguelperez Thanks so much for the kind PR, and especially thanks for the idiomatic test coverage!

Here are the properties being added, with links to the MDN docs for each:

• align-content
• align-items
• align-self
• flex
• flex-basis
• flex-direction
• flex-flow
• flex-grow
• flex-shrink
• flex-wrap
• justify-content
• order

Generally speaking, this PR looks good, I just want to spend some time to understand each of these properties (primarily to ensure we're not introducing any security risk) but based on the names and concepts I think that's unlikely to raise any issues.

Optimistically, I should have time to review and merge, and cut a release by the end of the week.

[miguelperez]

glad to help.

[flavorjones]

Shipped in https://github.com/flavorjones/loofah/releases/tag/v2.8.0!

Thanks again.