Add ability to programmatically change snackbar distance from borders

[Fs00]

Hi, I want to suggest a feature that would be extremely useful when you have a FAB and you want to avoid displaying snackbars over it.
It would consist of a function provided by withSnackbar() HOC that gives the ability to set the distance in px of the snackbars stack from the top/bottom border (an example of usage could be this.props.setVerticalDistanceFromBorder(8)), and maybe another one that resets this setting (e.g. this.props.resetVerticalDistanceFromBorder()).
I think that the developer should be given the ability to change it at any time because some views may have FABs and other may not. And if other snackbars are displayed while a new distance is set through this function, they should move up/down accordingly.
If some help is needed for the implementation of this feature, count me in. I need it for a school project, but I'm sure that it will be useful to other people too.

[iamhosseindhv]

@Fs00 Thanks for reporting this. This issue is similar to #57, and a new issue is created for it.