fix(config): not append empty module if no custom launcher/rep/prep

Just to get rid off the confusing debug message:
"DEBUG [plugin]: Loading inlined plugin (defining )."
feat: remove `karma` binary in favor of karma-cli

BREAKING CHANGE:
The `karma` module does not export `karma` binary anymore. The recommended way is to have local modules (karma and all the plugins that your project needs) stored in your `package.json`. You can run that particular Karma by `./node_modules/karma/bin/karma`. Or you can have `karma-cli` installed globally on your system, which enables you to use the `karma` command.

The global `karma` command (installed by `karma-cli`) does look for local version of Karma (including parent directories) first and fall backs to a global one.

The `bin/karma` binary does not look for any other instances of Karma and just runs the one that it belongs to.