[bs-css] compiles flex to flex-shrink, flex-grow, flex-basis

[thangngoc89]

Currently, bs-css only supports the simplest format of flex property: flex: 1 but there are more complex cases like:

flex: none                                            /* value 'none' case */
flex: <'flex-grow'>                                   /* One value syntax, variation 1 */
flex: <'flex-basis'>                                  /* One value syntax, variation 2 */
flex: <'flex-grow'> <'flex-basis'>                    /* Two values syntax, variation 1 */
flex: <'flex-grow'> <'flex-shrink'>                   /* Two values syntax, variation 2 */
flex: <'flex-grow'> <'flex-shrink'> <'flex-basis'>    /* Three values syntax */
flex: inherit

(from https://css-tricks.com/almanac/properties/f/flex/)

As per css best practice, it's recommended that you use flex shorthand property and avoid flex-shrink, flex-grow, flex-basis because flex will override all of them.

I hope that this ppx can automatically expand the flex property without compromising css's "best pratice" or use just unsafe if that's too much troubles.
What do you think?