Fix for malformed output.

Without this change, you can get malformed stuff like this:

  var foo = function() {
    return "foo";
  }var bar = function() {
    return "bar";
  }

By introducing a newline, a semicolon is implicitly inserted between statements
when necessary:

  var foo = function() {
    return "foo";
  }
  var bar = function() {
    return "bar";
  }

The indentation as a result of this commit is off, but I’m not sure how to fix
it because the build fails for me. (I tested this by editing dist directly.)

src/Module.js

@@ -44,7 +44,7 @@ export default class Module {
 			});
 
 			this.statements = ast.body.map( node => {
-				const magicString = this.magicString.snip( node.start, node.end );
+				const magicString = this.magicString.snip( node.start, node.end ).append( '\n' );
 				return new Statement( node, magicString, this );
 			});
 		} catch ( err ) {

src/Statement.js

@@ -248,7 +248,7 @@ export default class Statement {
 	}
 
 	replaceIdentifiers ( names ) {
-		const magicString = this.magicString.clone().trim();
+		const magicString = this.magicString.clone();
 		const replacementStack = [ names ];
 		const nameList = keys( names );