Ignore options.sourceFileName when printing

The root problem was that passing `options.sourceFileName` to the `Printer` would cause any new AST nodes (i.e. nodes that shouldn't have any source mapping) to appear to be taken from the so-named source file. If that confusion happened before the real `sourceContent` was set, the new AST node (typically the first node in the AST) would "win," providing the wrong content for that source file.

Closes #70.

cc @thomasboyt