Add command line application to sanitize HTML files

```text

Usage: html-cleaner [OPTIONS] INPUT [OUTPUT]

  A tool that modifies HTML to only keep allowed elements and attributes the

  same way that K-9 Mail does.

Options:

  -h, --help  Show this message and exit

Arguments:

  INPUT   HTML input file (needs to be UTF-8 encoded)

  OUTPUT  Output file

```

You can run this tool using the [html-cleaner](../../html-cleaner) script in the root directory of this repository. 

It will compile the application and then run it using the given arguments. This allows you to make modifications to the

[HTML cleaning code](../../app/html-cleaner/src/main/java/app/k9mail/html/cleaner) and test the changes right away.

apply plugin: 'org.jetbrains.kotlin.jvm'

apply plugin: 'application'

version 'unspecified'

application {

    mainClass = "app.k9mail.cli.html.cleaner.MainKt"

}

repositories {

    mavenCentral()

}

dependencies {

    implementation project(':app:html-cleaner')

    implementation "org.jetbrains.kotlin:kotlin-stdlib"

    implementation "com.github.ajalt.clikt:clikt:3.4.0"

    implementation "com.squareup.okio:okio:${versions.okio}"

}

package app.k9mail.cli.html.cleaner

import app.k9mail.html.cleaner.HtmlHeadProvider

import app.k9mail.html.cleaner.HtmlProcessor

import com.github.ajalt.clikt.core.CliktCommand

import com.github.ajalt.clikt.parameters.arguments.argument

import com.github.ajalt.clikt.parameters.arguments.optional

import com.github.ajalt.clikt.parameters.types.file

import com.github.ajalt.clikt.parameters.types.inputStream

import java.io.File

import okio.buffer

import okio.sink

import okio.source

@Suppress("MemberVisibilityCanBePrivate")

class HtmlCleaner : CliktCommand(

    help = "A tool that modifies HTML to only keep allowed elements and attributes the same way that K-9 Mail does."

) {

    val input by argument(help = "HTML input file (needs to be UTF-8 encoded)")

        .inputStream()

    val output by argument(help = "Output file")

        .file(mustExist = false, canBeDir = false)

        .optional()

    override fun run() {

        val html = readInput()

        val processedHtml = cleanHtml(html)

        writeOutput(processedHtml)

    }

    private fun readInput(): String {

        return input.source().buffer().use { it.readUtf8() }

    }

    private fun cleanHtml(html: String): String {

        val htmlProcessor = HtmlProcessor(object : HtmlHeadProvider {

            override val headHtml = """<meta name="viewport" content="width=device-width"/>"""

        })

        return htmlProcessor.processForDisplay(html)

    }

    private fun writeOutput(data: String) {

        output?.writeOutput(data) ?: echo(data)

    }

    private fun File.writeOutput(data: String) {

        sink().buffer().use {

            it.writeUtf8(data)

        }

    }

}

fun main(args: Array<String>) = HtmlCleaner().main(args)

#!/bin/sh

./gradlew --quiet ":cli:html-cleaner-cli:installDist" < /dev/null && ./cli/html-cleaner-cli/build/install/html-cleaner-cli/bin/html-cleaner-cli "$@"

include ':backend:jmap'

include ':backend:demo'

include ':plugins:openpgp-api-lib:openpgp-api'

include ':cli:html-cleaner-cli'