Refactoring: extract functions from checker.Visit
Generalize callReturnsError logic
Add --noblanks flag (no-op for now).

Also restructure checker.Visit to make room for blank identifier check.
Add check for assigning error to blank id.
Add extra test cases for blank identifiers
Mention -noblanks in README.
Rename -noblanks flag to -blank.
Merge pull request #22 from isbadawi/master

Checking assignment to blank identifier