Add API function for loading config file

[Victorystick]

As per a Gitter suggestion, we could move the logic to load arollup.config.js file out of the Rollup CLI code and into the library itself. That would make it easier for other people to use, and would decrease the amount of CLI-unique code by quite a bit. It may also make the code easier to test since we're only concerned with loading a set of options, and not actually bundling anything.

[TrySound]

Ref #828

[snuggs]

I concur. MUCH easier solution. Does this create a dependency PR with the cli? Curious where are the suspect lines within the respective repository. Curious to see how this plays out so I can begin contributing as much as I can ;-)

[fregante]

Duplicate of #761 too. May be best to merge all into one issue