feat: flag to keep file without toc untouched

add cli flag to prevent doctoc from adding toc to a file without toc.
this is good for CI or a combination with lint-staged.
with -p, doctoc will update toc if md has toc sectioon, otherwise doctoc keep it unmodified.

doctoc.js

@@ -16,7 +16,7 @@ function cleanPath(path) {
   return homeExpanded.replace(/\s/g, '\\ ');
 }
 
-function transformAndSave(files, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll, stdOut) {
+function transformAndSave(files, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll, stdOut, preserve) {
   if (processAll) {
     console.log('--all flag is enabled. Including headers before the TOC location.')
   }
@@ -26,7 +26,7 @@ function transformAndSave(files, mode, maxHeaderLevel, title, notitle, entryPref
   var transformed = files
     .map(function (x) {
       var content = fs.readFileSync(x.path, 'utf8')
-        , result = transform(content, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll);
+        , result = transform(content, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll, preserve);
       result.path = x.path;
       return result;
     });
@@ -79,7 +79,7 @@ var modes = {
 var mode = modes['github'];
 
 var argv = minimist(process.argv.slice(2)
-    , { boolean: [ 'h', 'help', 'T', 'notitle', 's', 'stdout', 'all' ].concat(Object.keys(modes))
+    , { boolean: [ 'h', 'help', 'T', 'notitle', 's', 'stdout', 'all' , 'p', 'preserve'].concat(Object.keys(modes))
     , string: [ 'title', 't', 'maxlevel', 'm', 'entryprefix' ]
     , unknown: function(a) { return (a[0] == '-' ? (console.error('Unknown option(s): ' + a), printUsageAndExit(true)) : true); }
     });
@@ -99,6 +99,7 @@ var notitle = argv.T || argv.notitle;
 var entryPrefix = argv.entryprefix || '-';
 var processAll = argv.all;
 var stdOut = argv.s || argv.stdout
+var preserve = argv.p || argv.preserve
 
 var maxHeaderLevel = argv.m || argv.maxlevel;
 if (maxHeaderLevel && isNaN(maxHeaderLevel) || maxHeaderLevel < 0) { console.error('Max. heading level specified is not a positive number: ' + maxHeaderLevel), printUsageAndExit(true); }
@@ -115,7 +116,7 @@ for (var i = 0; i < argv._.length; i++) {
     files = [{ path: target }];
   }
 
-  transformAndSave(files, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll, stdOut);
+  transformAndSave(files, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll, stdOut, preserve);
 
   console.log('\nEverything is OK.');
 }

lib/transform.js

@@ -106,7 +106,7 @@ function determineTitle(title, notitle, lines, info) {
   return info.hasStart ? lines[info.startIdx + 2] : defaultTitle;
 }
 
-exports = module.exports = function transform(content, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll) {
+exports = module.exports = function transform(content, mode, maxHeaderLevel, title, notitle, entryPrefix, processAll, preserve) {
   mode = mode || 'github.com';
   entryPrefix = entryPrefix || '-';
 
@@ -116,6 +116,10 @@ exports = module.exports = function transform(content, mode, maxHeaderLevel, tit
   var lines = content.split('\n')
     , info = updateSection.parse(lines, matchesStart, matchesEnd)
 
+  if (!info.hasStart && preserve) {
+    return { transformed: false };
+  }
+
   var inferredTitle = determineTitle(title, notitle, lines, info);
 
   var currentToc = info.hasStart && lines.slice(info.startIdx, info.endIdx + 1).join('\n')