add empty "index.js" to "rules/" directory

[donaldpipowitch]

Given palantir/tslint#2163 it would be nice, if the rules/ directory would contain an empty index.js. Then we could resolve the rules directory with Nodes module resolution which makes it easy to use custom rules in certain situations (symlinked configs, flat vs. non-flat installations...).

Motivation:

• extends: Allows me to use custom rules from other packages. I get all rules with default configs and can opt-out from rules. If new rules are added in new version I get them automatically.
• rulesDirectory : Allows me to use custom rules from other packages. I opt-in to use certain rules with my specific configs. If new rules are added in new version I don't get them automatically.

[adidahiya]

open to PRs