Should be able to pass in number of columns

Something like:

``` scss
$fig-max-col: 9;
$fig-col-width: ($fig-container - ($fig-gutter * ($fig-max-col - 1))) / $fig-max-col;
```

This would allow much better calculations for the widths of things. The math is only correct on eight columns, which is fine for the default but should be more precise if you want.

Start by locating the SCSS definitions for $fig-max-col, $fig-col-width, $fig-container, and $fig-gutter. Review how the current eight-column calculation is used; done means allowing the number of columns to be passed in so width calculations remain precise for values other than eight.

Written by the indexing model from the issue text.