stop formatting text if editor is not in normal mode

the autoformat function is called by vim when textwidth (if defined) is
exceeded. during these calls, the Fallback part of the fuction ends up
calling out to "normal! gww" which tends to reverse the last two
characters.

this is caused by how gww works, in relationship to how formatexpr is
called during insert mode -- gww tries to bring the cursor back to where
it was before, but the cursor position did not exist in the first place
(since the last typed character was not inserted into the buffer yet
before calling formatexpr)

This change works around the issue by entirely disabling autoformatting
when not in normal mode. It means that only using gq in normal mode will
be able to reformat the text.

autoload/puppet/format.vim

@@ -1,6 +1,13 @@
 "
 " Simple format using puppet's l:indents and align hashrockets function
 function! puppet#format#Format() abort
+  " only allow reformatting through the gq command
+  " (e.g. Vim is in normal mode)
+  if mode() !=# 'n'
+    " do not fall back to internal formatting
+    return 0
+  endif
+
   let l:start_lnum = v:lnum
   let l:end_lnum = v:lnum + v:count - 1
   " Don't modify indentation or alignment if called by textwidth. We'll only