Add imports to example Clikt program

[m0xb]

The example program in the readme and quickstart doc page (https://github.com/ajalt/clikt/blob/master/README.md and https://ajalt.github.io/clikt/quickstart/) don't include imports. Trying to compile fails with "Unresolved reference: CliktCommand".

It looks like the following imports are needed:

import com.github.ajalt.clikt.core.CliktCommand
import com.github.ajalt.clikt.parameters.options.default
import com.github.ajalt.clikt.parameters.options.help
import com.github.ajalt.clikt.parameters.options.option
import com.github.ajalt.clikt.parameters.types.int
import com.github.ajalt.clikt.parameters.options.prompt

I see an old issue mentioning the same thing, but it's closed now: https://github.com/ajalt/clikt/issues/335