Add roots_display_sidebar filter

Add filter to sidebar display to have the ability to change when it is displayed in child themes and other circumstances.

https://github.com/retlehs/roots/issues/767
Removed extra space before apply filters
Merge pull request #771 from gruvii/patch-1

Add roots_display_sidebar filter
Merge pull request #160 from retlehs/master

Added roots_display_sidebar filter