Add support for the space character for the show-invisibles plugin

[zeitgeist87]

This patch adds a bit of position magic, so that the invisibles
are displayed on top of the original character. This allows for
the inclusion of the space character.

The result looks like this:

[mAAdhaTTah]

Nice!

[mAAdhaTTah]

I dig it. I think the saturation could be turned down a bit. Compared to how I have it in my editor, I think they're a bit dark and hard to distinguish from the characters surrounding them. I changed it to this:

.token.space:before {
  color: hsl(24, 20%, 35%);
  content: '\00B7';
}

[mAAdhaTTah]

Actually, that only looks better on the darker themes (which is what I tend to use). Not sure what the balance is here.

[LeaVerou]

Nice!