This change makes it so that all of Anvil's existing configuration options can be set via a `gradle.properties` or a `-P_______` command line option.

For any setting, Anvil take the first non-null value out of:
1. Settings in the Gradle DSL:
   ```kotlin
   anvil {
     generateDaggerFactories.set(true)
   }
   ```
3. Properties set via the command line like:
   ```sh
   ./gradlew build -Pcom.squareup.anvil.generateDaggerFactories=true
   ```
4. Properties specified in a project-level, root-project-level, or `~/.gradle/`-level `gradle.properties`:
   ```properties
   com.squareup.anvil.generateDaggerFactories = true
   ```
5. Hard-coded default values (never null)

Practically speaking, this makes it possible to *locally* feature-toggle things (like the upcoming incremental fix) without touching a project's build logic.