Make clicktarget for subnav links larger.

Closes #10.

Also possible to move the `margin-right: 30px` from the `li` to padding on the `a` (0 15px), which would make the target even more clickable, but I didn't want to mess with that too much, especially since we'd also need to look at the responsive views. Happy to experiment further, if warranted.
Merge pull request #91 from charliepark/patch-1

Make clicktarget for subnav links larger.