Fix #1: Prevent split when opening on start

When using itchy_startup, we still obeyed itchy_always_split. Now we
pass force_split as an argument so the caller can decided whether they
want a split. No split on startup, otherwise use itchy_always_split. (We
still always split if the buffer is modified.)

Using itchy_startup makes ctrlp's default behaviour to not replace
plugin-created windows more annoying, so you can disable that behaviour
for itchy:

    let g:ctrlp_reuse_window = 'Scratch'
Remove numbering from help

Numbering just adds more work when editing and since everything is
linked, it doesn't do much for navigation.
Add ctrlp compatibility information to docs

I needed to change ctrlp_reuse_window when I started using
itchy_startup. Add to documentation so other people can find this
solution.
Replace itchy_startup with ScratchNoSplit

Instead of trying to open a scratch buffer on start, just provide a way
to open a scratch buffer without forcing a split.
Add Saveas command to save a scratch buffer

Scratch buffers give errors if you try to save them, so provide a
command to turn off the scratch settings and pass the bang and file
through to built-in saveas.

Saveas command only exists in scratch buffers.

Addresses #2. There's no cmdalias because it's too awkward. (I actually
always use 'save' instead of 'saveas', but 'sav' is also valid. I don't
want to have an explosion of aliases, so I'll leave it as a straight
command for now.)