Add custom rules directory to build process.

Build any rules found in oclint-rules/rules/custom if that directory exists.
This lets people add rules specific to their own projects that may not be relevant to
oclint users in general.

We're hoping to integrate oclint into our development workflow and that includes checking for things that are only relevant to our codebase. It seemed to me that these things shouldn't be in the default install of oclint since no one else is interested in them. This mechanism gives us an easy way to add those rules ourselves and build them from a clean checkout of oclint.

To be clear, we're hoping to open source any rules that we write, we just don't think anyone else wants some of them, especially not by default.