Error when attempting to run with showdown 1.3.0

Thanks for this helpful extension :-).

I'm using it with showdown 1.3.0 via grunt-showdown (actually via [ferllings/grunt-showdown](https://github.com/ferllings/grunt-showdown) because the original repo doesn't work with showdown 1.3.0). When I try to use showdown-toc I get the following error:

> Error in 2 extension-> sub-extension 0: property "type" must be a string, but undefined given

This does not appear to be grunt-showdown-specific because if I comment out showdown-toc it works fine; also I am using other showdown extensions such as [showdownjs/prettify-extension](https://github.com/showdownjs/prettify-extension).

I am using a config like this in Gruntfile.js:

``` javascript
showdown: {
all: {
options: {
customExtensions: ['showdown-prettify', 'showdown-toc']
},
files: [{
expand: true,
src: 'tmp/book/**/*.md',
ext: '.html'
}]
}
},
```

Start by reproducing the failure with the customExtensions configuration shown in Gruntfile.js, using showdown-toc with showdown 1.3.0 and grunt-showdown. Then inspect the showdown-toc extension entry point and compare its setup with the working prettify extension. Done means the configuration runs without the "property \"type\"..." error while both extensions remain enabled.

Written by the indexing model from the issue text.