Add line_height config option

[ataractic]

Closes #166

Makes the line height configurable in pixels instead of being hard-coded to font_size * 1.2.

Thought of other options as well:

• Add option for automatic text height based on the font size (like before this pull request)
• Add option for separately setting the spacing between the results and the query line

[j0ru]

Hey, thanks for the pull request! Unfortunately I'm currently in the middle of moving countries, but I'll review it as soon as I have a stable network connection and a minute to think about it ;)

[j0ru]

Hm, I'm not a fan of changing a relative value into an absolute one, because that would impact everyone that uses a non-default font size. If you'd make the value optional and default back to the original behavior of multiplying the font size, I'd be okay with it