Added post numbers to generation

cms.py will calculate the next post number when generating a new post,
and pass it to make_post.py. It does it in a fairly clumsy way by just
counting the number of directories in the posts/ directory, so a better
way should be found eventually.
Added initial HTML saving ability

HTML is now saved in the post's new folder, but the actual original contents of
the post are not saved. This needs to be added soon - probably in a
separate file called save_post.py.

Editing should also be added, so that
already written posts can be modified. This could fairly easily be
accomplished by adding some functionality to make_post.py