Make type: block the default for actions
Since 99% of our rules have an action type of block, make that the default.
```mson
- trigger:
- url-filter: somedomain.ext
```
This also means that both the trigger and action sections are now optional:
```mson
- url-filter: somedomain.ext
```
And, to support the 99% use case that we want to block a third-party domain, let’s add a transformation to the Builder so we can simply support Blockdown rules in the following form:
```mson
somedomain.ext
```
All of the above Blockdown rules will compile to the following proper complete WebKit content blocking rule in the JSON:
{
"trigger": {
"load-type": [
"third-party"
],
"url-filter-is-case-sensitive": true,
"url-filter": "^[^:]+://+([^:/]+\\.)?somedomain\\.ext[:/]?"
},
"action": {
"type": "block"
}
}