Add missing $is-input argument

Fixes Sass compile errors if you've included $is-input in custom button classes with Foundation 5.2. Related to https://github.com/zurb/foundation/issues/4871
[FIX] @import "type" not necessary
Allow custom error class name for Abide
!important statement missing for tables

Visibility classes for tables are not working as expected in all cases due this missing `!important`.
If it's missing they got `display: inherit !important` which results in rendering issues.
Fixed border styles for inputs. 

Now we can set unique border properties for all sides, with use variables.
Added font stack variables
Use a normal for loop

The alternative being combining `for…in` with `hasOwnProperty` I thought using the vanilla loop was preferable.
docs(typo): you to your


Change "makes you site feel faster" to "makes your site feel faster."
warning for libsass compataibility
Fixes empty z-index property to be 'auto'.

Fixes a bug that causes a z-index property to have no value. Rather than using 'null' (which results in no value for the property), 'auto' is used instead.

This bug is a problem if you use a build process like Uncss to trim unused CSS. The build will fail as the empty z-index property isn't valid.
warning for libsass compataibility
Minor typo in comments.

Minor, but it bugged me.
warning for libsass compataibility
Set base color on panels, fixes #5768
small change in warning
fix bug with radio and checkbox validation

This was introduced with #5738.

The statement was in the wrong line. This fixes a bug where the validation of radios and checkboxes was not recognized in the final validation result.

Now it works great :)
Merge pull request #5773 from jonathanmelville/master

Sass compile of _reveal.scss results in empty z-index property
Merge pull request #5769 from markwtaylor/master

docs(typo): you to your
Merge pull request #5748 from andy-saint/font-stacks

Added font stack variables
Merge branch 'master' of https://github.com/zurb/foundation