Use non-breaking space for unit separator

[nox-4O4]

Currently, a plain space is used for separating unit and number string. This may lead to the unit wrapping to a new line:

While this can easily be worked around using CSS styling or a custom wrapper, it would be nice if this module used a non-breaking space to separate the unit.

[sindresorhus]

I'm willing to add an opt-in option for this, but non-breaking space can cause other problems. For example, if you use it in a CLI app and some tool tries to parse the output, they would not expect a non-breaking space.