Iglistkit: Center aligned flow layout

[MarvinNazari]

Hi thanks for this amazing library! loving it so far, the only issue that i have right now is that i need a center aligned collection view layout, is there any way we could add some sort of alignment to the new IGListGridCollectionViewLayout so that we could have both left and center alignment?

[zhubofei]

@jessesquires I can take on this, if you guys approve this api.

[Sherlouk]

@zhubofei Are you still interested in working on the extra alignment ability of your grid layout?

[zhubofei]

@Sherlouk Yes, I'm gonna post a PR by the end of this week 😊.

[rnystrom]

Closing since we have a new layout inbound in #450. Once that's landed we can revisit if this is still needed.