Fix applying 'wrap' to ignored windows

Set the wrap setting instead of a local variable. l:wrap defines a new
variable that modifies nothing. &l:wrap is the window-local 'wrap'
configuration value.

With g:golden_ratio_wrap_ignored == 0, ignored windows are unwrapped.
With g:golden_ratio_wrap_ignored == 1, ignored windows are wrapped.

Update documentation to reflect the behavior.
Fix restoring 'wrap' setting with no autocmd

Store g:golden_ratio_saved_wrap before applying wrap if not already
stored. This ensures that whatever the user's 'wrap' setting was before
golden_ratio modified it will be restored.

Check for existence of the variable, not the nonexistent vim option.

Tested with golden_ratio_wrap_ignored = 0 with and without
g:golden_ratio_autocommand enabled and with and without 'wrap' enabled.
Main window always had the user's setting and ignored windows always had
'nowrap'.