feat: make cterm colors configurable

_cterm globals has been added for configuring the extra colors
in 256-color terminal emulators.

README.md

@@ -203,6 +203,13 @@ You can customize each of Srcery's colors, to customize say the red color:
 let g:srcery_red = '#FF0000'
 ```
 
+Inside a 256-color terminal emulator, additional colors are configurable
+via setting the corresponding variable with the \_cterm suffix, e.g.
+
+```vim
+let g:srcery_hard_black_cterm = 232
+```
+
 Refer to the [table](#TUI) for a full list of color variables, hexes and more.
 
 This will only work on `set termguicolors` and in gVim, to override terminal

autoload/srcery.vim

@@ -69,38 +69,74 @@ if !exists('g:srcery_orange')
   let g:srcery_orange='#FF5F00'
 endif
 
+if !exists('g:srcery_orange_cterm')
+  let g:srcery_orange_cterm=202
+endif
+
 if !exists('g:srcery_bright_orange')
   let g:srcery_bright_orange='#FF8700'
 endif
 
+if !exists('g:srcery_bright_orange_cterm')
+  let g:srcery_bright_orange_cterm=208
+endif
+
 if !exists('g:srcery_hard_black')
   let g:srcery_hard_black='#121212'
 endif
 
+if !exists('g:srcery_hard_black_cterm')
+  let g:srcery_hard_black_cterm=233
+endif
+
 if !exists('g:srcery_xgray1')
   let g:srcery_xgray1='#262626'
 endif
 
+if !exists('g:srcery_xgray1_cterm')
+  let g:srcery_xgray1_cterm=235
+endif
+
 if !exists('g:srcery_xgray2')
   let g:srcery_xgray2='#303030'
 endif
 
+if !exists('g:srcery_xgray2_cterm')
+  let g:srcery_xgray2_cterm=236
+endif
+
 if !exists('g:srcery_xgray3')
   let g:srcery_xgray3='#3A3A3A'
 endif
 
+if !exists('g:srcery_xgray3_cterm')
+  let g:srcery_xgray3_cterm=237
+endif
+
 if !exists('g:srcery_xgray4')
   let g:srcery_xgray4='#444444'
 endif
 
+if !exists('g:srcery_xgray4_cterm')
+  let g:srcery_xgray4_cterm=238
+endif
+
 if !exists('g:srcery_xgray5')
   let g:srcery_xgray5='#4E4E4E'
 endif
 
+if !exists('g:srcery_xgray5_cterm')
+  let g:srcery_xgray5_cterm=239
+endif
+
 if !exists('g:srcery_xgray6')
   let g:srcery_xgray6='#585858'
 endif
 
+if !exists('g:srcery_xgray6_cterm')
+  let g:srcery_xgray6_cterm=240
+endif
+
 " }}}
 " Options {{{
 
@@ -190,15 +226,18 @@ let g:srcery#palette.bright_cyan    = [g:srcery_bright_cyan, 14]
 let g:srcery#palette.bright_white   = [g:srcery_bright_white, 15]
 
 " xterm colors
-let g:srcery#palette.orange         = [g:srcery_orange, 202]
-let g:srcery#palette.bright_orange  = [g:srcery_bright_orange, 208]
-let g:srcery#palette.hard_black     = [g:srcery_hard_black, 233]
-let g:srcery#palette.xgray1         = [g:srcery_xgray1, 235]
-let g:srcery#palette.xgray2         = [g:srcery_xgray2, 236]
-let g:srcery#palette.xgray3         = [g:srcery_xgray3, 237]
-let g:srcery#palette.xgray4         = [g:srcery_xgray4, 238]
-let g:srcery#palette.xgray5         = [g:srcery_xgray5, 239]
-let g:srcery#palette.xgray6         = [g:srcery_xgray6, 240]
+let g:srcery#palette.orange         = [g:srcery_orange,
+                                    \  g:srcery_orange_cterm]
+let g:srcery#palette.bright_orange  = [g:srcery_bright_orange,
+                                    \  g:srcery_bright_orange_cterm]
+let g:srcery#palette.hard_black     = [g:srcery_hard_black,
+                                    \  g:srcery_hard_black_cterm]
+let g:srcery#palette.xgray1         = [g:srcery_xgray1, g:srcery_xgray1_cterm]
+let g:srcery#palette.xgray2         = [g:srcery_xgray2, g:srcery_xgray2_cterm]
+let g:srcery#palette.xgray3         = [g:srcery_xgray3, g:srcery_xgray3_cterm]
+let g:srcery#palette.xgray4         = [g:srcery_xgray4, g:srcery_xgray4_cterm]
+let g:srcery#palette.xgray5         = [g:srcery_xgray5, g:srcery_xgray5_cterm]
+let g:srcery#palette.xgray6         = [g:srcery_xgray6, g:srcery_xgray6_cterm]
 
 " }}}
 " Setup Emphasis: {{{

doc/srcery.txt

@@ -141,6 +141,11 @@ You can customize each of Srcery’s colors, to customize say the red color:
 >
 	let g:srcery_red = '#FF0000'
 <
+Inside a 256-color terminal emulator, additional colors are configurable
+via setting the corresponding variable with the _cterm suffix, e.g.
+>
+	let g:srcery_hard_black_cterm = 232
+<
 Refer to the |srcery-color-table| for a full list of color variables, hexes
 and more.