Allow flags to be set with boolean values

TypeDoc like TypeScript takes several command line arguments that don't have a value. Those currently cannot be set using grunt-typedoc. Let's simply take all options with a boolean value as flags:

```
typedoc: {
    build: {
        options: {
            module: 'commonjs',
            out: 'doc/',
            name: 'My documentation',
            target: 'es5',
            excludeExternals: true,
            noLib: false
        },
        src: 'src/**/*.ts'
    }
},
```