Disable leading newline

[c4eater]

This change adds a new TermOption, "has_leading_newline".

When this option is set to false (default is true), TermLock::ensure_height() would not add a leading newline (=the separator between the original line contents and the new contents) to the output anymore.

When it is set to true, there is no change in ensure_height()'s behavior (the leading newline is present).

I'm using this option and setting it to false in my version of skim (https://github.com/c4eater/skim/tree/disable-leading-newline) because the output looks better without the leading newline.

Here are some screenshots of skim with and without the leading newline: If has_leading_newline is false:

If has_leading_newline is true (default):