Make ApplicationCall.respondHtml and ApplicationCall.respondHtmlTemplate accept suspendable functions

### Ktor Version
1.1.3

### Ktor Engine Used(client or server and name)
ktor-server-servlet

### JVM Version, Operating System and Relevant Context
1.8, macOS

### Feedback
I am working on an application that uses `ktor-html-builder` combined with asynchronous API calls, and I am having a hard time getting clean and concise code because I am not allowed to `.await()` within my HTML blocks.

Right now, I am forced to do this:

``` kotlin
Routing.routes() {
get("/") {
val firstResultDeferred = async { firstCall() }
val secondResultDeferred = async { secondCall() }

val firstResult = firstResultDeferred.await()
val secondResult = secondResultDeferred.await()

call.respondHtml {
body {
p {
+ "First result: $firstResult"
}
p {
+ "Second result: $secondResult"
}
}
}
}
}
```

Instead, I would like to be able to do this:

``` kotlin
Routing.routes() {
get("/") {
val firstResult = async { firstCall() }
val secondResult = async { secondCall() }

call.respondHtml {
body {
p {
+ "First result: ${firstResult.await()}"
}
p {
+ "Second result: ${secondResult.await()}"
}
}
}
}
}
```

Start at the ktor-html-builder implementations of ApplicationCall.respondHtml and ApplicationCall.respondHtmlTemplate, the entry points named in the report. Trace how their HTML-builder lambdas are defined and verify both APIs support await() inside the blocks while preserving the shown HTML output.

Written by the indexing model from the issue text.