Add --data-dir as a flag instead of passing it via --k3s-extra-args

## Expected Behaviour

It would be convenient to have a flag for `--data-dir` in the k3sup install and join commands.

## Current Behaviour

A longer flag is required:

`--k3s-extra-args="--data-dir=/mnt/ssd/k3s"`

## Possible Solution

You don't need an SSD to test this, the command just installs k3s into a custom path for its data.

Make --data-dir output additional arguments to k3s-extra-args, as per other flags we already have that work this way.

Bear in mind that there is a `setupAdditionalServer` and `setupAgent` function, both of which work in a very similar way.

## Context

Thought of this whilst looking into #373

There also appears to be a bug in the join command which means extra args are ignored due to the ordering of string concatenation?


Related to: https://github.com/alexellis/k3sup/pull/388