[Feature Request] Custom canvas width in landscape mode

[rainux]

Is your feature request related to a problem? Please describe.
Recent iPhone models has a 19.5:9 display ratio, while reading in landscape mode this display ratio will result a view which is too narrow in height.

Describe the solution you'd like
We can allow users to configure the left and right padding for landscape mode of Reading View, so users can got a view balanced in height & width.

Describe alternatives you've considered
I didn't find any alternatives.

Additional context
I've implemented this with fixed padding, but since I'm not very familiar with Swift, didn't make it as a option in Reading Settings yet. If you like this idea, I'll try fully implement it and send PR.

[tatsuz0u]

Thanks for your advice.

A horizontal padding adjustment feature does seem helpful while reading in vertical direction and landscape mode. We will add it to our todo list.

cc @chihchy