Fix neovim GUI detection

Regression was introduced in 211c438 and actually disabled this plugin
for neovim entirely, whoopsie and sorry for that!

The neovim GUI detection is a single dumpster fire:

Why do we have to listen to autocmds?
Why can't we just use has('gui_running') at any point, like in vim and
gVim?
Why can't we just use UIEnter and must check an extra flag?

Who in their right mind would design such a convoluted clusterfuck,
especially if vim already has such a sane API for this, BUT NO WE HAVE
TO BE FUCKING SPECIAL IN NEOVIM.
All the explanations are just lazy: Oh, you can't have gui_running set
on startup, because plugins are sourced before the GUI attaches?
Oh ok, let's just make it hard for our users then, so that we don't have
to redesign our shit spaghetti code, what a great idea!?!

The VimEnter autocmd already is there for all TUI startup needs. Why
does UIEnter trigger for TUI startup too???

To fix this mess, we check for the chan flag on UIEnter, which is 1 if
we are in a GUI and 0 if we are in a TUI.

Fixes #30

autoload/tpipeline.vim

@@ -119,7 +119,7 @@ func tpipeline#initialize()
 
 		call tpipeline#util#set_size()
 		au VimResized * call tpipeline#util#set_size()
-		au UIEnter * call tpipeline#state#restore()
+		au UIEnter * if v:event['chan'] | call tpipeline#state#restore() | endif
 	endif
 endfunc