Add option g:haskell_indent_let_no_in

This addresses some of the grievances in neovimhaskell#85. Namely, we give users the
option to change how indentation works with `let` statements.

Current behavior:

    -- g:haskell_indent_let
    let x = 0
    >>>>y = 0

    let x = 0 in
    >>>>x

    -- g:haskell_indent_in
    let x = 0
    >in x

    -- ??? (conflated with g:haskell_indent_let)
    let x = 0
    >>>>x

New behavior:

    -- g:haskell_indent_no_in
    let x = 0
    >>>>x

So for example, those who omit the 'in' on the first line and any 'let'
on the second line can opt to have no indentaton:

    --- g:haskell_indent_no_in = 0
    let x = 0
    fooBar x

Author: jez

indent/haskell.vim

@@ -34,9 +34,18 @@ endif
 if !exists('g:haskell_indent_let')
   " let x = 0 in
   " >>>>x
+  "
+  " let x = 0
+  "     y = 1
   let g:haskell_indent_let = 4
 endif
 
+if !exists('g:haskell_indent_let_no_in')
+  " let x = 0
+  "     x
+  let g:haskell_indent_let_no_in = 4
+endif
+
 if !exists('g:haskell_indent_where')
   " where f :: Int -> Int
   " >>>>>>f x = x
@@ -208,6 +217,9 @@ function! GetHaskellIndent()
   "
   " let x = 1
   " >>>>y = 2
+  "
+  " let x = 1
+  " y 2
   if l:prevline =~ '\C\<let\>\s\+.\+$'
     if l:line =~ '\C^\s*\<let\>'
       let l:s = match(l:prevline, '\C\<let\>')
@@ -219,11 +231,16 @@ function! GetHaskellIndent()
       if s:isSYN('haskellLet', v:lnum - 1, l:s + 1)
         return l:s + g:haskell_indent_in
       endif
-    else
+    elseif l:line =~ '\s=\s'
       let l:s = match(l:prevline, '\C\<let\>')
       if s:isSYN('haskellLet', v:lnum - 1, l:s + 1)
         return l:s + g:haskell_indent_let
       endif
+    else
+      let l:s = match(l:prevline, '\C\<let\>')
+      if s:isSYN('haskellLet', v:lnum - 1, l:s + 1)
+        return l:s + g:haskell_indent_let_no_in
+      endif
     endif
   endif