[O] [feature request] support content level in #+startup

[Zhang Haijun]

The manual says there are following options deals with the initial 
visibility of the outline tree:

overview         top-level headlines only
content          all headlines
showall          no folding of any entries
showeverything   show even drawer contents

I want to show headlines whose level is less than 3(level 1 and 2). Can 
you support this? For example, support the following option:

content:2         show headlines whose level is no larger than 2