Browseold: Fix bug introduced in last change, add filter-out

Bug: Forgot to change second instance of 'write' to 'set nomodified.'
Filter-out: CTRL-S was to filter so only lines matching a pattern
    remain. The reverse can be useful too, so CTRL-V now filters so only
    lines NOT matching a pattern remain.
Browseold: Refactor a tiny bit.
Make Browseold a bit prettier

Use a statusline to show "Browsing old files" instead of putting it in
the buffer. Also, use cursorline!