Support official HTML reporter

Since the HTML reporter is now integrated in ktlint 0.36.0, it should be added to the configurable ones.

Right now

ktlint {
    reporters {
        reporter "html"
    }
}

produces

> Cannot convert string value 'html' to an enum value of type 'org.jlleitschuh.gradle.ktlint.reporter.ReporterType' (valid case insensitive values: PLAIN, PLAIN_GROUP_BY_FILE, CHECKSTYLE, JSON)

[smuga]

Before it’ll be available, you still can use HTML reporter by adding this:

customReporters {
    "html" {
        fileExtension = "html"
        dependency = "com.pinterest.ktlint:ktlint-reporter-html:0.36.0"
    }
}

[Tapchicoma]

@ImMathanR We are planning to do new release this week, worth case beginning of the next week.