Remove LineNr, Todo background color

This is something I can never make up my mind about. On the one hand, it
is nice to have a clear delineation between the buffer's contents and
the line numbers, but there just isn't enough padding around the line
numbers to make it look neat.

The previous coloration of Todo made it invisible on a line with
CursorLine highlighting.

Additionally, link SignColumn to LineNr.

colors/noctu.vim

@@ -1,7 +1,7 @@
 " noctu.vim - Vim color scheme for 16-color terminals
 " --------------------------------------------------------------
 " Author:   Noah Frederick (http://noahfrederick.com/)
-" Version:  1.3.0
+" Version:  1.4.0
 " --------------------------------------------------------------
 
 " Scheme setup {{{
@@ -29,7 +29,7 @@ hi SpellCap            ctermfg=10    ctermbg=NONE  cterm=underline
 hi SpellRare           ctermfg=11    ctermbg=NONE  cterm=underline
 hi SpellLocal          ctermfg=13    ctermbg=NONE  cterm=underline
 hi NonText             ctermfg=8
-hi LineNr              ctermfg=8     ctermbg=0
+hi LineNr              ctermfg=8     ctermbg=NONE
 hi CursorLineNr        ctermfg=11    ctermbg=0
 hi Visual              ctermfg=0     ctermbg=12
 hi IncSearch           ctermfg=0     ctermbg=13    cterm=NONE
@@ -57,7 +57,7 @@ hi User7               ctermfg=15    ctermbg=12
 hi User8               ctermfg=15    ctermbg=11
 hi User9               ctermfg=15    ctermbg=8
 hi! link CursorColumn  CursorLine
-hi! link SignColumn    Normal
+hi! link SignColumn    LineNr
 hi! link WildMenu      Visual
 hi! link FoldColumn    SignColumn
 hi! link WarningMsg    ErrorMsg
@@ -75,7 +75,7 @@ hi Underlined      ctermfg=4   cterm=underline
 hi Type            ctermfg=4
 hi String          ctermfg=9
 hi Keyword         ctermfg=2
-hi Todo            ctermbg=11  ctermfg=0        cterm=bold
+hi Todo            ctermfg=15  ctermbg=NONE     cterm=bold,underline
 hi Function        ctermfg=4
 hi Identifier      ctermfg=7   cterm=NONE
 hi Statement       ctermfg=2   cterm=bold