Fix for missing wp:term_id data by supplying an increment for tag.id

I ran into a bug converting my blog.  The error was:

```
wp2ghost/wp2ghost.js:105
				tag.id = parseInt(item['wp:term_id'][0]);
				                                    ^
TypeError: Cannot read property '0' of undefined
    at /Users/dave/Sites/repos/wp2ghost/wp2ghost.js:105:41
    at Array.forEach (native)
    at /Users/dave/Sites/repos/wp2ghost/wp2ghost.js:100:9
    at fn (/Users/dave/Sites/repos/wp2ghost/node_modules/async/lib/async.js:582:34)
    at Object._onImmediate (/Users/dave/Sites/repos/wp2ghost/node_modules/async/lib/async.js:498:34)
    at processImmediate [as _immediateCallback] (timers.js:330:15)
```

The issue is that I had a few tags, but the tags didn't have data for `wp:term_id`.  I've added a check for this, and if it's missing I use the array index of the tag instead.  With this fix I was able to convert everything.