[Feature] Add config option for padding space to left

[0-issue]

It would be nice to have a configuration option to add some space from the left border of terminal. It would make web pages look more readable.

[novaTopFlex]

For some users, w3m may be more readable with left padding. However, there could then be issues with the lack of right padding. Without the padding on the left and right edges, reading might not be comfortable for all users of w3m.