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
!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
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
Revert "Merge branch 'istrasoft-PR-dropdown-fit-bounds'"

This reverts commit feb991b977c0185e6947731945cdd000c2b97e33, reversing
changes made to 429b8e8b02a438bd363c6867fa02b2a4fad53285.
$reveal-max-width variable missing from _settings.

Although referenced in the Reveal docs, the $reveal-max-width variable is missing from the _settings.scss file.