Support for Gradle configuration cache

[davidburstrom]

As of version 0.6.0, this is the only problem in the way of supporting the Gradle configuration cache:

⌄cannot serialize object of type org.gradle.api.internal.project.DefaultProject, a subtype of org.gradle.api.Project, as these are not supported with the configuration cache. ?
⌄field$this_run of com.appmattus.markdown.plugin.MarkdownLintPlugin$apply$1$1
⌄bean of type com.appmattus.markdown.plugin.MarkdownLintPlugin$apply$1$1
⌄fieldthis$0 of com.appmattus.markdown.plugin.MarkdownLintPlugin$apply$1$1$2
⌄bean of type com.appmattus.markdown.plugin.MarkdownLintPlugin$apply$1$1$2
⌄fieldaction of org.gradle.api.internal.AbstractTask$TaskActionWrapper
⌄bean of type org.gradle.api.internal.AbstractTask$TaskActionWrapper
⌄fieldactions of org.gradle.api.DefaultTask
task:markdownlint of type org.gradle.api.DefaultTask

[athkalia]

enabling configuration cache via
org.gradle.unsafe.configuration-cache=true
I get

Execution failed for task ':markdownlint'.
> Cannot invoke "org.gradle.api.Project.getBuildDir()" because "this.this$0.$this_run" is null