Added flexbox properties and values

[larslaade]

I added all current flexbox properties and values:

.flexbox {
    align-content: flex-start;
    align-content: flex-end;
    align-content: center;
    align-content: space-between;
    align-content: space-around;
    align-content: stretch;
    align-items: flex-start;
    align-items: flex-end;
    align-items: center;
    align-items: stretch;
    align-items: baseline;
    align-self: auto;
    align-self: flex-start;
    align-self: flex-end;
    align-self: center;
    align-self: baseline;
    align-self: stretch;
    display: flex;
    display: inline-flex;
    flex: 0 1 auto;
    flex-basis: auto;
    flex-direction: row;
    flex-direction: row-reverse;
    flex-direction: column;
    flex-direction: column-reverse;
    flex-flow: row nowrap;
    flex-flow: column wrap-reverse;
    flex-grow: 0;
    flex-shrink: 1;
    flex-wrap: nowrap;
    flex-wrap: wrap;
    flex-wrap: wrap-reverse;
    justify-content: flex-start;
    justify-content: flex-end;
    justify-content: center;
    justify-content: space-between;
    justify-content: space-around;
    order: 1;
}