Better behavior for #31, autoformat lists in Markdown

For tpope's markdown syntax, restored standard autoformat behavior for unordered lists, which is generally pretty good.

Ordered lists had annoying behavior for autoformat, so simply disabling autoformat if detected when insert is entered. This isn't ideal, but it's not clear what else can be done now.

Fixed bug for PFormatToggle, where it wasn't properly disabling Vim's autoformat.

Provided help in README for mapping key to toggle autoformat.

autoload/pencil.vim

@@ -121,8 +121,10 @@ fun! s:maybe_enable_autoformat() abort
       endfo
     en
   en
-  " disallow enable if start of previous line is in blacklist
-  if l:line > 1
+  " disallow enable if start of previous line is in blacklist,
+  " (To avoid problem of autowrap screwing up adding a new item
+  " to a list.)
+  if l:okay_to_enable && l:line > 1
     let l:prev_stack = synstack(l:line - 1, 1)
     for l:sid in l:prev_stack
       if len(l:sid) > 0 &&
@@ -152,7 +154,6 @@ fun! pencil#setAutoFormat(af) abort
       au InsertEnter <buffer> call s:maybe_enable_autoformat()
       au InsertLeave <buffer> set formatoptions-=a
     aug END
-    let b:last_autoformat = l:nu_af
   el
     sil! au! pencil_autoformat * <buffer>
     if l:nu_af && !l:is_hard
@@ -162,6 +163,7 @@ fun! pencil#setAutoFormat(af) abort
       return
     en
   en
+  let b:last_autoformat = l:nu_af
 endf
 
 " Create mappings for word processing