fix(filter.template): usage parameters
test(link filter): add test of the filter processing
feat(code filter): allow language to be passed
feat(code custom tag): allow language to be passed to the tag

You can now specify the language of the code block as a parameter to the
tag:

```
{% code 'js' %}
function () {
}
{% endcode %}
```
chore(ngdoc): remove unused code
feat(doc-extractor): add project relative path to docs
fix(base template): use relative doc path in link to github