Allow custom function in releaseRules

[jedwards1211]

There have been many JS tools that created some kind of static configuration DSL, and those DSLs always end up being janky to work with. Which is why so many tools have ended up allowing configuration to live in .js.

Since semantic-release already allows a .js config file, it's strange that releaseRules doesn't accept a custom function already.

But if it did it would be a better workaround for the lingering issues in #122.