[feature] add options for width and height preview

[gmr458]

This is related to #15 , in my case the problem was the width and height of the preview, I solved it by modifying these lines:

These modifications that I made probably won't work on other screen sizes, other font sizes and other telescope settings like layout_config and it's a bit "dirty" to modify the source code of the extension, so I think it would be nice to have these options available.

[dharmx]

@gmr458 I don't know if you're interested but, I already added those options on top of what tembokk's fork provides!