Harp 0.17 does not allow blank lines to appear between filters and their
content. This change ensures that any blank lines that could appear have
been trimmed inside Nunkjucks.
Closes#3325
This means that we can now run just the d.ts file generation by running:
```bash
gulp docs/typings
```
In addition the type definition generation was messing with the other docs tasks
so separating it also fixes problems there.
You can generate docs for comsumption by the angular.io website by running:
```bash
gulp docs/angular.io
```
The generated docs can be found in `dist/angular.io`
Peter Bacon Darwin
Alex Wolfe