Allow easier variables overwriting

Right now to customize LESS variables there are two options:

- Edit `less/bootstrap.less`. This forces the user to modify the Bootstrap source code, but this option is not always available (e.g. when Bootstrap is included through Bower or Composer). Also, changing third party code doesn't seem a very good practice.
- Copy and edit `less/bootstrap.less`. Here Bootstrap isn't touched but it requires copying all those imports (and updating their paths), this makes updating a little more painful.

With the proposed changes one can do this:

```css
@import "components/bootstrap/less/variables.less";
@import "variables.less"; // Bootstrap variables overwritten here
@import "components/bootstrap/less/styles.less";
```

`less/bootstrap.less` is functionally equivalent to current implementation so it could also be backported to version 2.x.