Add conditional_modifiers rule

The [thoughtbot style guide](https://github.com/thoughtbot/guides/tree/dc293558279e3cc0d8365355334b334b477a7728/style#coffeescript) has a rule stating the following.

> Avoid conditional modifiers (lines that end with conditionals).

In order to facilitate automatic enforcement of this rule I'm adding it as a CoffeeLint rule. The rule is disabled by default, since this rule is presumably quite specific to thoughtbot.