This PR implements startColumn option in Babel parser. Like startLine, the startColumn indicates the starting column position of the input string in the source code. For example, consider a wrapper

function (exports, require, module, __filename, __dirname) { [code] }

where [code] is a string from which Babel parser will generate AST. A user will expect the generated location info correctly maps to the location in the wrapped code, in this case, he/she should pass

{
  "startColumn": 60
}

to Babel parser so that the line 1 column 0 at [code] maps to line 1 column 60 in the wrapped source. Not that unlike startLine, the startColumn is 0-based, its default value is 0.