PR: Add option to specify the output directory for generated .html.ts file

Hi,
I need to be able to have my generated .html.ts file stored in a separate directory from the original template html files. My solution is to provide a htmlOutDir option, where all the generated files are stored.
```javascript
            htmlWithHtmlOutDirTest: {
                test: true,
                src: ['test/htmlOutDir/reference.ts','test/htmlOutDir/src/bar.ts',
                    'test/htmlOutDir/src/foo.ts',
                    'test/htmlOutDir/generated/**/*.ts'],
                html: ['test/htmlOutDir/**/*.tpl.html'],
                reference: 'test/htmlOutDir/reference.ts',
                htmlOutDir: 'test/htmlOutDir/generated',
                out: 'test/htmlOutDir/out.js'
            },
```

One criticism I have of my own solution is that the files are stored flat (all in the same directory) so collisions may occur. I would be happy to hear any alternatives/alterations to my solution