Documentation and renaming of "blankies" into somethign a bit easier to ...

...find.

* Rename SplitExpression* to `SbtParser` denoting that is parses .sbt files
* Adds a few todos
* Document APIs for internal usage.

Review by @ajozwik  cc> @eed3si9n 
I agree, sort of.   It's useful to have a method which just takes the raw streams or "virtual file" so we can do in-memory settings (like `set` command in sbt), where multiple settings could be in a string.   Right now we do not do that so you are correct.

I agree we should work on simplifying API now that the feature is pretty stable.   I can add a TODO here about removing the need for lines and reading in splitExpressions.  For now, as it's a private API, it's ok to refactor this at any time.  I"m more concerned with the exposed public APIs *and* ensuring this area is documented for the next user group that wants to work on it :)