Add notification height option

[wfdewith]

Since the line_height option doesn't work like it is supposed to, because it sets the line spacing rather than the height of the line itself (see #104), I would like to have an option to set the height per notification.

I am willing to do this myself, but will it be accepted as a pull request?

[knopwob]

Yes, I would merge such a pull request. Just try to keep the default the same way it is now, so nothing changes in behaviour for people who don't configure this option.

[wfdewith]

I believe you can close this issue and #104 now

[hughdavenport]

I believe this can be closed now?

Cheers,

Hugh