Created by: schoenburg
margin-*: auto is an invaluable tool when using flexbox positioning,
and with Bootstrap 4 supporting flexbox, it would be good
to have it built in as a base utility to avoid the need for messy
one-off classes added by end users.
This adds auto as a spacer value to use with the spacing utility.
.m-t-auto is an example of the class being used. With this new class,
the explicit .m-x-auto class is no longer needed as it is generated
like the others.
Documentation updated with note adding auto