Highlight integer literals using exp. notation

We extend the existing rules for identifying a numeric in a simply way -- allow a `L` character following a 'numeric' value. Hence, values like `1E5L`, `0.1E5L`, `1.5E5L` will now be highlighted.

Modifying the highlight rules in this way obviates the explicit integer highlighting rule.
Merge pull request #86 from kevinushey/patch-2

Highlight integer literals using exp. notation