Allow variables to be overriden by user

By adding 'unless' to each variable like:

```css
$black =                 #000 unless $black is defined;
$grayDarker =            #222 unless $grayDarker is defined;
$grayDark =              #333 unless $grayDark is defined;
$gray =                  #555 unless $gray is defined;
```

This way the user can do something like:
```css
$linkColor = #F00;
```
before importing bootstrap