Allow disabling heuristics on a per-filetype basis

References: #29

doc/sleuth.txt

@@ -55,14 +55,23 @@ CONFIGURATION                                   *sleuth-configuration*
 To override detection and force options for a particular file or directory,
 use a |modeline| or EditorConfig (https://editorconfig.org/), respectively.
 
+                                                *g:sleuth_{filetype}_heuristics*
+To disable the heuristic algorithm for a given 'filetype', assign a false
+value to a variable named like g:sleuth_{filetype}_heuristics:
+>
+        let g:sleuth_gitcommit_heuristics = 0
+<
+                                                *g:sleuth_heuristics*
+Alternatively, one can disable heuristics by default, and opt-in for
+individual file types:
+>
+        let g:sleuth_heuristics = 0
+        let g:sleuth_perl_heuristics = 1
+<
                                                 *g:sleuth_automatic*
 Automatic detection of buffer options can be disabled entirely with:
 >
         let g:sleuth_automatic = 0
-<
-or:
->
-        let b:sleuth_automatic = 0
 <
                                                 *g:sleuth_no_filetype_indent_on*
 Sleuth forces |:filetype-indent-on| by default, which enables file-type

plugin/sleuth.vim

@@ -455,7 +455,7 @@ function! s:DetectHeuristics(into) abort
   if has_key(detected, 'patterns')
     call remove(detected, 'patterns')
   endif
-  if empty(filetype)
+  if empty(filetype) || !get(b:, 'sleuth_automatic', 1) || empty(get(g:, 'sleuth_' . filetype . '_heuristics', get(g:, 'sleuth_heuristics', 1)))
     return detected
   endif
   if s:Ready(detected)
@@ -589,10 +589,10 @@ endfunction
 augroup sleuth
   autocmd!
   autocmd BufNewFile,BufReadPost * nested
-        \ if get(b:, 'sleuth_automatic', get(g:, 'sleuth_automatic', 1))
+        \ if get(g:, 'sleuth_automatic', 1)
         \ | exe s:AutoInit() | endif
   autocmd BufFilePost * nested
-        \ if (@% !~# '^!' || exists('b:sleuth')) && get(b:, 'sleuth_automatic', get(g:, 'sleuth_automatic', 1))
+        \ if (@% !~# '^!' || exists('b:sleuth')) && get(g:, 'sleuth_automatic', 1)
         \ | exe s:AutoInit() | endif
   autocmd FileType * nested
         \ if exists('b:sleuth') | silent call s:Apply(s:DetectHeuristics(b:sleuth), s:safe_options) | endif