plugin: Add new onBufferOptionChanged callback

This can become handy in the moment a plugin needs to react on e.g. changed
file type.

internal/buffer/settings.go

@@ -1,13 +1,23 @@
 package buffer
 
 import (
+	luar "layeh.com/gopher-luar"
+	ulua "github.com/zyedidia/micro/v2/internal/lua"
 	"github.com/zyedidia/micro/v2/internal/config"
 	"github.com/zyedidia/micro/v2/internal/screen"
 )
 
 func (b *Buffer) SetOptionNative(option string, nativeValue interface{}) error {
+	oldValue := b.Settings[option]
 	b.Settings[option] = nativeValue
 
+	if oldValue != nativeValue {
+		_, err := config.RunPluginFnBool(b.Settings, "onBufferOptionChanged", luar.New(ulua.L, b), luar.New(ulua.L, option), luar.New(ulua.L, string(oldValue.(string))), luar.New(ulua.L, string(nativeValue.(string))))
+		if err != nil {
+			screen.TermMessage(err)
+		}
+	}
+
 	if option == "fastdirty" {
 		if !nativeValue.(bool) {
 			if !b.Modified() {

runtime/help/plugins.md

@@ -55,6 +55,10 @@ which micro defines:
 * `onBufferOpen(buf)`: runs when a buffer is opened. The input contains
    the buffer object.
 
+* `onBufferOptionChanged(buf, option, old, new)`: runs when a option of the
+   settings of a buffer has changed. The input contains the buffer object,
+   the option, the old and the new value.
+
 * `onBufPaneOpen(bufpane)`: runs when a bufpane is opened. The input
    contains the bufpane object.
 

runtime/plugins/linter/linter.lua

@@ -141,6 +141,33 @@ function onSave(bp)
     return true
 end
 
+function onBufferOptionChanged(buf, option, old, new)
+    if option == "filetype" then
+        if old ~= new then
+            ft = old
+            for k, v in pairs(linters) do
+                local ftmatch = ft == v.filetype
+                if v.domatch then
+                    ftmatch = string.match(ft, v.filetype)
+                end
+
+                local hasOS = contains(v.os, runtime.GOOS)
+                if not hasOS and v.whitelist then
+                    ftmatch = false
+                end
+                if hasOS and not v.whitelist then
+                    ftmatch = false
+                end
+
+                if ftmatch then
+                    buf:ClearMessages(k)
+                end
+            end
+        end
+    end
+    return true
+end
+
 function lint(buf, linter, cmd, args, errorformat, loff, coff, callback)
     buf:ClearMessages(linter)