Feature request: support `join` option

[scottrippey]

I would like a "join" option, so that multi-line strings could be both dedented and joined into a single line.
Example:

example = dedent(' ', `
  This is a
  multiline string
  that is joined with spaces
  instead of newlines.
`); // "This is a multiline string that is joined with spaces instead of newlines."

A little backstory, I actually have my own dedent function implemented in several projects, and use it constantly! I found this repo when thinking about publishing my dedent. I'm glad to see it already exists!
This 'join' feature is something I use quite often, too, so I'm hoping its something I could add to this project. Let's discuss?